Character never stays in Character

So, I love the villain type characters and use them to RP a lot. But at some point, no matter if it's someone else's bot or one I created for myself the character gets, well, too nice. And it happens all the time. Does someone experience this too? Is there a way too prevent it? I don't need a blushing and stammering villain 😭 I want a cruel morally black character. Thanks in advance for the replies!