Why would he feel so bad about killing them?
Considering not only the fact that they tormented his brother to death, they were also drug dealing criminals who probably were involved in the number of other serious crimes as well, why would he feel so bad about killing them in the end, when at first, all he ever felt was how necessary and right it was?
I mean, they WERE the bad guys right.
In various action movie instances like Die Hard, Commando, Lethal Weapon and many of the James Bond films, the lead heroes had no remorse towards killing bad guys, some of them even did it out of revenge rather than self defense, but here...
At the end especially, the lead character feels bad about killing them, even goes as far as to say it wasn't worth it for his brother and claims that if the guy doesn't help him kill himself, he is going to kill more people - good people instead.
So why would he gain so much conscience after killing the thugs that brutalized his brothers and were drug dealing criminals as well?
The greatest trick the Devil has ever pulled was convincing the world he didn't exist!