I don’t know about , because I can’t write that. But to write a character in internal dilemma is difficult, especially if he’s conscious about it.
So you have a character whose morals are being overpowered, he changes from one thing to another - the latter being something he would have perceived as vile if he had been who he was earlier.
The biggest key point is to not show “what he would have thought if he was able to think straight”. He isn’t thinking straight, it doesn’t matter what he would’ve thought before he changed. What matters is now.
You can - in a moment of weakness - mention that he cannot recognize himself anymore, if he really changes that much, but don’t let it become big.
The best way to make the change is to show who he was before.
If we’re talking “something so vile” as in something criminal, have him encounter something criminal on TV, in real life, in discussion with friends to voice his opinion on it before you slowly trap him into your plot and what changes him and corrupts his morals.

At least that’s what I do and how I think it’s done best. Because it’s better to show than tell and characters are fluid things that changes and have complex thoughts, ideas and discussions and sometimes morals are useless.