I agree with @aSofaNearYou here. I feel sorry for the stepson in the sense that it’s certainly not his fault that the set-up at his mother’s home is so poor that not only are social services involved, they’re also looking to remove him from her custody.
However, I absolutely draw the line at a helpless baby being put at risk of serious injury or even death at the hands of the 9-year-old stepson, regardless of how traumatised he is thanks to his upbringing. Plenty of kids grow up in dysfunctional homes without resorting to threatening/inflicting violence on an infant. This is really extreme and, although I feel sad for the trauma the 9-year-old has experienced, in no way should the safety of the baby be compromised to accommodate that trauma. That includes coming to live with the OP and her partner full-time.
OP, if I were in your shoes, I’d be keeping a log of every single threat/action your stepson takes to harm your child (including dates and times). Alert social services of your concerns and tell them you’re keeping this log. If they do decide to remove your stepson from his mother’s care then they can’t place him with you if they know that he poses a risk to his younger sibling. In your log, note down every time you speak to social services too, including the name and job title of the person you spoke to.
In the meantime, when your stepson comes to stay, I absolutely echo other posters who have said never to leave your stepson alone with your baby, even if it’s for 30 seconds while you fetch a parcel from the postman. I would also recommend that you have a camera baby monitor in the baby’s nursery that is set up to record any movement and also lock/alarm the nursery door. It sounds extreme but if your stepson realises he’s not being allowed to be left alone with your baby and he is indeed hellbent on causing him harm then he may well resort to extreme measures such as creeping into the baby’s room after the adults have gone to bed.
Oh and to address the title of your thread - it’s perfectly fine for you to dislike a child if they’re trying to cause your son harm! Not all children are likeable unfortunately and I for one don’t think you’re at all to blame for feeling the way you do.