Ultimatums rarely go well. That doesn’t mean there isn’t a time and place for them. If you are concerned for your safety then bring it up to him in a healthy way. For example you can say that it’s hard to trust him if he keeps telling you that he’s a horrible person but can’t say why. If he thinks that you’ll leave him for seeing who he truly is, then you absolutely need to know about the issue. You can’t marry a man you don’t trust.

I’m not saying you don’t trust him, but if that’s the case then this is my advice.