The poisoning of Mario's beer went quite unnoticed by the rotund plumber. In fact, he hadn't even noticed Luigi took one of his beers in the first place. If he had, he probably would've hit him and yelled again, but that's beside the main point. Nobody would even care what he'd do to Luigi anyway, so it's not worth bringing it up.
Luigi brought the can of beer, along with the bags of picked up groceries, into the kitchen. What seemed to be the casual unloading of groceries was, in fact, a much darker act. Tucked away, hidden from sight in one of Luigi's overalls pockets, was a small vile of Shyanide poison. The serum was obtained from eBay at a fairly reasonable price. At first, Luigi wasn't completely sold on the deal, but after hearing about its 2 hour delivery from across the country and its free shipping, he couldn't refuse the offer. The package was ordered to arrive when the idea first struck him at the grocery store, timed perfectly so that it would arrive just around the time he would get home. Still, it amazed him how Mario hadn't even noticed a package arrive at the door. How pathetic, Luigi thought to himself.
The plumber put away some the groceries to appear like he was busy - not that Mario would even notice. But still, being cautious was never a bad thing. The small canister of Shyanide was removed form the side pocket of Luigi's denim, and the little cork on the top of it was popped open with a little bah!
Luigi grimaced as he heard the sound, terrified Mario might've heard it. He anxiously peered into the living room, only to find his elder brother half-asleep on the nasty couch, mumbling something offensive about Floridians. A major sigh of relief came from the soon-to-be murderer. It looked like things were going Luigi's way for once in his life, and that put a smile on his face. The television remained turned on with the sports still playing, giving Luigi cover for making any other sounds in case Mario decided to wake up.
This made him more comfortable to open the beer can next, now ignoring the loud hiss of carbonation. He stared down at the frothing alcohol, and sighed. When Mario would drink he'd often become violent; much more hostile then today's encounter, for sure. Often Luigi would suffer the consequences of Mario's drinking. This tiny little can was one of the main factors of the constant abuse he regularly endured, and it infuriated him. Luigi almost wanted to crush the can out of spite, but held his temper. If he destroyed the can, his plan would be ruined.
Instead, he took a deep breath and poured several drops of poison into the beer can. Moments later, he returned into the living room, placing the can of infected beer onto Mario's coffee table, replacing an already opened can with this new one. This way, when Mario awoke to drink some more - because Mario doesn't care about hangovers - he would drink the poison without even realizing it.
In just a few hours after, his muscles would weaken, his head would spin, and his breathing would become labored. At first, he would push it off as a cold or a hangover, not thinking anything unusual. However, by the time he'd realize something would be wrong, it'd be too late. The poison would have contaminated his entire nervous system, and thus, his entire body. He would be dead within minutes after that, and Luigi would be there to witness his final gasps of breath. The satisfaction Luigi would feel be immeasurable as he would watch his struggling brother try to desperately figure out what happened to him, never receiving an answer.
Then Luigi would be free of his brother, free of abuse, and free of living the shadow of Mario Mario. He could finally create a name for himself, and become the new hero of the Mushroom Kingdom. Oh, how perfect life would be for him! And after all the suffering endured by that obese red man, Luigi deemed himself deserving of this fate.
"Sleep well, bro." Luigi mumbled to the fat Italian on his way passing by, wishing on him one last peaceful dream. It would be the man's last, after all, so Luigi thought he should enjoy it. A large smile crept up on his face, knowing know all he had to do was watch and wait.