The original version of this story was entirely an alternate universe/history, a Flash fanfiction, in which Zoom took notice of Snart. The circumstances of that alternate history result in some OOC moments, as I deem them relevant to the plot. (I don't usually do that, so bear with me.)

This slightly modified version is technically a Legends of Tomorrow/Flash crossover, in which Rip Hunter shows Snart what would have happened if he left the team prematurely or was never recruited. Thus the framing story here.

The title, "Alternate Universe," is because of both the alternate circumstances that separate this story from canon... and because of Zoom and a certain meta-human assassin's origins from another Earth.

Legends team members copyright to DC, CW, etc.
Majummed copyright to me.

"I'm afraid I haven't been very honest about why I've recruited you," Rip admitted.

"Shocker," Leonard muttered. "First, you tell us we're to be... 'heroes.'" He fairly sneered the word. "Then, we're nothing; if something goes wrong on your mission, and we don't make it back, our absence won't change the future. So what fresh lie are you going to feed me this time?"

"Er..." Rip hesitated. "See, um... Well, we've seen how difficult it can be to change the future. That time wants to happen. But I've found, sometimes, that removing an individual from the time stream, displacing them a little, can make an unexpected difference."

"Is there a point to all this?"

"Well..."

—ALTERNATE UNIVERSE—

"Leonard? What the hell are you doing? Let go of him!" Sara raced into Rip's quarters and tried to drag Leonard away from the captain, whom he was presently trying to strangle.

Mick waded into the fight and bodily picked up his partner. "I'm all for killing Rip," he said, "but shouldn't we be drawing straws or something?"

"Okay, what did you do to piss him off?" Sara asked Rip once Leonard was pulled off.

"Me?" Rip panted. "Why do... you assume... it's my fault?"

"Because it's clearly safer than blaming Mr. Snart?" Martin suggested from the doorway. He shrugged. "Face it, you're a lot less likely to pull a gun over it."

"He threatened Lisa!" Leonard growled, his face twisted into uncharacteristic rage. "He said if I didn't do what I was told—"

"You have a perfect memory," Rip snapped. "If you'd use it, you'd realize that is not what I said!"

"So what did you say?" Sara asked.

"I said... I said if he wanted to protect his sister, he needed to stay on this ship. Stay on the mission."

Sara rolled her eyes. "Uh-huh. Hardened criminal, a little too much experience doing things he doesn't want just to keep her safe, and... you didn't expect him to find that remark threatening. To maybe hear it the same way he's heard it from the rodent he calls a father."

"You'd better explain yourself, Captain," Mick said from where he was struggling to hold Leonard in the corner. "'Cause I'm about to let go."

Rip threw his hands in the air. "If all of you would let me explain..." Several pairs of eyes watched him expectantly. Most of the crew had joined Martin in the door to watch the show. "Fine. Like I... began to explain to Mr. Snart, I wasn't entirely truthful about why I recruited the lot of you."

"Yeah, you told us we were supposed to be heroes," Kendra said.

"And then you said we'd have no impact on history," Martin added. "So what is the truth?"

"Well, that second one is technically true for some of you," Rip admitted. "That is, that you have no impact on the future I'm familiar with. If we change the timeline, that, too, would change."

Mick loosened his hold on Leonard just a little. Just enough to suggest a threat if Rip didn't hurry up and finish explaining.

"But in Mr. Snart's case..." Rip added quickly. "Mr. Snart has a rather... impressive influence on the timeline, in any version of events in which he is back in his own time. The exact same influence his sister has within this timeline." He took a deep breath and forced himself to meet Leonard's eyes. "I'm sorry, Mr. Snart. I should have told you this before, when you and Mr. Rory demanded to use the jump ship. Hell, I should've told you back when I recruited you. But I've done extensive research on all of your timelines and this one—the one in which you remain here—this is the only one in which your sister has that influence."

"What happens to her in the other timelines?" Martin asked.

"A metahuman," Rip muttered. "I believe you and Mr. Jackson know him as Zoom."

"Zoom?" Mick repeated. "Sounds familiar."

"Another speed-freak," Leonard said. He shrugged Mick off, without ever once breaking eye contact with Rip. "Broke the Flash, took him out of commission for a while."

"Oh. That freak."

"So what's supposed to happen?" Leonard asked. "Zoom thinks he can recruit me to help him against the Flash, and Lisa's left out in the cold?" He smirked. "I don't mind sharing credit with Lisa if that's what you're worried about."

"Hardly," Rip said. "Zoom knows about your arrangement with the Flash. He knows the Flash worked hard to save your sister's life when your own father was prepared to kill her. And he knows that when the Weather Wizard broke you and the Trickster out of prison, you chose to warn the Flash against them. He does not try to recruit you. If you show up in that time again, he decides you're a problem to eliminate, and he recruits Majummed—" he gave Sara a significant look as Leonard began trembling uncontrollably "—a metahuman from his own world, to kill you."

"That's a stupid name," Mick said.

"That's a League name," Sara replied.

"Correct," Rip said. "He's from the League of Shadows, what we would call the League of Assassins. And I dare say they've never known a more sadistic bastard."

"But... but what does this have to do with Lisa?" Leonard whispered.

"Gideon, would you please play the footage?" Rip asked.

—ALTERNATE UNIVERSE—

Two shadows met in the darkness.

"You may destroy him however you like," one of the shadows was saying in Zoom's rasping voice. "But bring the Flash to me, alive and unharmed."

The other shadow, wearing garb similar to the League of Assassins, bowed in reply. "As you command." The assassin hesitated. "Might I... consider other contracts as well? Things have become rather dull at home since you took charge, but surely this world has not met an assassin of my skill."

Zoom nodded. "As you like. Just do not let your entertainment interfere with my contract. Do not fail me, Majummed, or what happened to your sister will be nothing to my wrath."

"As you command," the assassin repeated in a slightly shaky voice. Then he spun on his heel and left.

Technically, this story is still meant to be entirely alternate universe, with no—relevant—connection to my other fics, let alone to canon.
However, Snart's reaction to the name Majummed (and Sara and Rip noticing said reaction as the rest of the team—Snart included—does not) does tie it somewhat vaguely to one of my other fics-in-progress...

As to Snart's "perfect memory," it has been suggested on assorted character pages that the CW's rendition of Snart may have eidetic memory. I'm sure I don't use that quite accurately within my fics, but I do allude to it now and again.

Timing... is questionable.
Technically there is no place in canon where there's actually time to have this discussion (let alone after Mick and Snart demanded to take the jumpship late-season, which demand you'll note Rip very specifically refers to here), another detail that makes this an alternate universe.
But the way I figure it, for this to work, Rip would have to pull an "oh good you're still here" moment immediately after Martin sent Jefferson off in the jump ship (because, "oh crap I should've told him this instead of letting him leave")... and Ray Palmer is currently holding the idiot ball as he goes round and round against Vandal Savage.

Edited to modify naming in narrative... and similar reasons.