It wasn't meant to end this way; he hadn't intended to die yet.
As Regulus was pulled to his death by Voldemort's inferi, he regretted it all.
What had he done to his poor house-elf? Not only would Kreacher be devastated at losing Regulus, he would also have no choice but to continuously punish himself until the locket was gone, and it was almost impossible to destroy a horcrux. Regulus didn't even know how to, and no house-elf would have the intelligence to figure it out, even if they knew it was a horcrux they were looking at. Which Kreacher did not.
And then there was that Dung Fletcher. Thieving snake; always selling things on.
What if it ended up in the wrong hands? What if the coward Tom Riddle got it back, and Regulus had died in vain?
He regretted everything.