Am I a bad person?

I have been with my partner for 2 and a half years now. My partner has 2 kids from his previous relationship, and I really struggle to 'like' them. Don't get me wrong, I'm never horrible to them they are kids!! If anything they absolutely love me and won't leave me alone at times, so I have to lock myself away sometimes or just go out to get away. I have tried to accept them thinking I will grow to live with the fact he has 2 kids, but it doesn't seem to get any better. I love my partner so much, but if I can't accept his kids surely I have to walk? He is such a good dad, and thinks the world of his 2 girls. I don't know if it's because we lost our first child, and now struggle to have another. I used to feel jealous, now I don't. They are spoilt brats who speak to people like dirt, expect to have EVERYTHING all the time. And I'm not aloud to say anything because they are NOT my kids so I don't have a say in how they are disciplined.....SO basically, does anyone else have mutual feelings like this about their partners kids?? Shared stories are appreciated :)