He just wants to ignore it, and I'm not convinced it will stay ignorable.
You are correct, it will not.
I guess I need to know what I should do if William presents himself again. Should I try and get to know him, ask him what makes him different? Or should I just ask him for my husband back?
Yes, I would definitely get to know him. Don't ask him what makes him different, but ask him who he is, when he spent time in the body. Ask him if he's willing to tell you anything about what happened to him. I wouldn't ask for your husband back unless it's absolutely necessary, like he has a pressing appointment or a job to go to. I would try not to let him know that you favor one of the two over the other, though that may be difficult. If William is to trust you, it's is important not to reject him. William is a critical part of your husband, likely a part that saved his life in some way. It's likely that he was at one time necessary to your husband's physical, mental or emotional survival. He deserves credit for this. If he isn't dangerous, you might thank him for all he's done (vaguely, because he may not like your husband).
Of course you sense that DID and alters cannot remain hidden forever without an eventual breakdown such as you you recently witnessed. It sounds probable that William holds some nasty abuse. Perhaps your husband doesn't want to know about it, or is aware to some degree (because he knows of "Billy") but actively is not willing to face his alter, because that means facing the abuse.
I would talk to William when he arrives and treat him as who he presents himself to be, because that's who he is. Let him know you would like to work with him and help him but you're limited in what you can do and that you are also loyal to your husband and want to keep his entire system stable. I assume William knows about your husband? If William ever presents dangerously again, it may be wise to to leave. When your husband tries to communicate to you to come back, you could use the danger you felt as a bargaining tool for him to seek therapy. It may feel challenging but it could also be self-preservation. Obviously these are just ideas I'm throwing out, you're the owner and best judge of your relationship.
There may be an approach you could use by way of analogy to speak to your husband about the potential danger of continuing to avoid his alter and his DID. You might very indirectly talk to him about a situation that parallels the perils of avoiding an uncomfortable problem. For example, you might talk about someone who didn't file their taxes for years, then lost their business and went to jail. You might know of someone who had a lump somewhere that eventually progressed to cancer before medical help was sought. Better if someone other than you were to tell such a tale it could resonate (though I wouldn't necessarily suggest setting someone up to do that!) Or rent a movie where that is the theme.
In the meantime, I would learn as much about DID as possible and I hope you'll feel comfortable to ask further questions here.