I don't know how many dramas I've seen where the ML is supposed to be dark, villainous, or evil, yet always turns into a flop in "critical" moments. Any drama where the ML is truly dark? Like he has no problems killing secondary female leads who consistently try to manipulate things and put the female lead in danger or secondary male leads who attempt to get close the FL, and actually DOES.... like geez... every drama I've seen which has pictured this type of ML, in the end he's just a husky.

I'd argue that Wu Lei's character, Ling Buyi, in Love Like The Galaxy is the perfect type you're looking for. He is a very gray protagonist who is viciously loyal to his close circle and the FL and cold/ruthless to all else. He doesn't become contrived or wishy-washy later; his goals remain, his personality remains, and he doesn't lose sight of who he is, even while in love.  He's one of the few ML's I've ever watched who doesn't become watered down once he falls in love. He still cuts people down with a simple glare and is still a powerful and intimidating person throughout. Easily top 5 dramas I've ever watched. Drama is split into 2 parts on MDL, but it's 1 season overall (56 episodes).  https://mydramalist.com/687167-xing-han-can-lan

Burning Ice - so this drama has 2 male leads. One is the detective and he is grey but not per say evil. Just grey. The 2nd male lead though? One of the best character developments I have ever seen. He is good in the beginning but his morals change quite extremely. 

Maybe, Lucifer/The Devil, plus its japanese remake  Maou.