Oh OP, my heart goes out to you.
The fact is that some people when they really aren't coping do get extreme and terrifying thoughts. People who think he's dreadful for having them have just been lucky enough to not be in that situation. Most doctors and most professionals in this field know that. The key for them is knowing who is horrified at the thoughts and asking for help, and who is talking as if it's realistic. It's probably a good thing that your husband told you this at all and a sign that there is a lot of trust in you.
Your family urgently needs outside help. Immediately. He's not coping at all (I get it, two ND children too, one with violent meltdowns, and an ex-husband who got his rocks off on undermining me).
Please @daddywoe go to the doctor immediately. Be -honest- about how bad it is. Those are some scary thoughts he's having, and like others who've been in this situation I get it, but I also know he needs help, and soon.
@RedRedCapris suggested speaking to the safe guarding lead person at school. Please do this. @SpidersAreShitheads posts on this thread are gold.
A SEN parenting course may be really valuable for him and you. Probably essential.
In practical terms for now, it's actually not bad if he steps back mentally from her. With a difficult autistic child I actually got to the point of imagining a mask over my face and smoothing it out, and treating him as a super-difficult member of the public that I had responsibility for. It helped at the time - could that mental trick help him? it's a deliberate stepping back but it's better than just walking away and you can put the mask off later when it's not needed.
Also, you need to try to build in a bit of time each day for yourself, which may seem overwhelming right now but if you're dealing with the children every day on your except for a short time before bed, that's a big strain on you. Whatever gives you peace, try to build it in 2 x a day for ten minutes at least.
I looked up some sites but you can look up more to get external help. Tell them it's an emergency as your husband has had brief fantasies of hurting her.
https://www.pdasociety.org.uk/
https://www.childautism.org.uk/
https://www.youngminds.org.uk/ (There's a helpline for parents at the bottom)
your local council may be able to help (emphasis, 'may').
And last but really not least, the doctor.
It can be utter hell with an aggressive and/or violent child. It -will- get better, but it can be a long hard road.
Also - you can skim over and dismiss a lot of the more unhelpful or stupid comments like 'leave him'. How, exactly, are you meant to cope with 2 NT children and a very very challenging 3rd ND child on your own? That's a recipe for a breakdown in 3 months.
There's no point to people saying 'he's a bad dad'. There's no prize for this self-indulgent moral game-playing, can only make you feel bad (what's the point of that?) and it makes no practical difference to solving the situation. He's tried, and he's at the end of his tether, and good or bad he's still her dad. At this point the only thing that's going to help is accepting that he is where he is and he can't cope, and needs help.