That's a tough spot. I think if it was me, I'd be as honest as his age allows. Does he need details? At first, probably not. Something like "Your daddy wasn't very nice to us, so I moved us away from him to make sure you were safe and happy. Maybe one day he'll realize how great you are and try to be a better person." Then maybe as he gets older and wants to know, you can divulge more details if he needs them. IDK, just a suggestion, but I've not dealt with this first hand. I'm sure there are SMs who have dealt with this and will have some amazing advice for you!

Wow.. I have no advice, but I'm really proud of you for leaving!

is he going to get violent if you cut him out of the kids lives completely?

Honest as the kids can handle it is good.  You would tell a 10 year old something different than a 5 year old.  your STBXH most likely does have some mental issues because he can not deal with reality, so you aren't lying if you say he was sick and needed help.

Yeah basically what @etherial said.  

My ex threw a MICROWAVE at me once.  He actually slid it down the entire length of the kitchen counter and I was standing at the end. I  hopped back in very last second knick of time to avoid it landing on me and this was a big microwave.  Kids were 6 and six months old at the time, it woke the baby and the six year old came running out of his room to see what was up.   exH said "Oh I didn't mean that to go in your direction..."  like hell! 

That was his MO, to throw stuff at me then deny he was throwing it at me.  And it happened really infrequently so I "overlooked" it.  The last time was right before I told him to get out and it was years later, and he basically tossed his entire computer desk at me, with the tower and monitor and everything.  It hit my thigh and gave me a huge bruise the size of a dessert plate that was raised up 3/4 of an inch and uggggggly.    He promptly went to bed (!) and stayed there for almost 2 days and when he woke up he looked at my leg and said "Wow, that's a nasty bruise!" and his tone was totally that of someone who did not know how the bruise got there.  It was creepy.

Needless to say he had/has mental health problems but he never would follow through on treatment or taking meds or anything and that is pretty much what I told the boys.  I didn't offer up info but when they asked, or when they were sad because he wouldn't call them on the phone I would explain that he was sick and needed help but had a hard time admitting that he was sick so he wasn't getting help.  I always told them that he loved them and just wasn't taking care of himself enough to show it.  Basically what @etherial said.

It's been almost 10 years and they have come to their own realizations about him now, at 14 and 20.  The 20 year old is a little bitter about it and says things like "gee, mom, if I sit around and drink and play video games and call it depression will YOU support ME forever?" (cuz my ex's parents still support him).  My 14 year old, who is my ex's biological child (DS20 is not), is just kind of like "Well Dad sucks but I'll hang out with him when he asks because I love him even though he's messed up..."   he is much more accepting now.   And my ex has never been violent or aggressive towards the kids. 

Neglectful, yes, but that's why I didn't let them go to him when they were younger unless his parents were going to be around.  Now they are old enough that they can go visit him a little an I know they are old enough to take care of themselves if he is doing his usual (which is usually nothing).  But he is "good" when they go visit.  Thanks to his parents he has plenty of cash when they are there so they eat out all the time and go to video game conventions and sporting events and whatever.

The point of my long post is, I was always careful to not talk shit about him but I didn't lie for him either and while my youngest had a very hard time at first, they've both eventually learned on their own what the deal is with him and have come to terms with it.

beachmommy said:

@charlotte_sometimes - OMG - I had an ex that threw a microwave at me too!  It hit my arm and left a huge bruise...believe it or not, that was my final "straw".  IDK why it took me so long to leave, but once he threw my "foodsource" it was OVER.   

I can laugh about it now bc THAT crazy man sleeps with the worms.... What goes around comes around!

Should have been my final straw but my DS was just an infant and I hung in there like a fool for another 4.5 years!   What's with the "throwing stuff at you is not the same as hitting you" bullcrap?  Really.   When the throwing person is 6'3" 180, it can be worse than just hitting you! 

Good grief!  When my ex threw the computer and computer desk at me and made that huge bruise on my leg and my IL's asked what happened, I told them with no sugar coating.  MIL says to me "Oh yeah, he always did have a temper, used to put holes in the walls when he was a teenager..,."   as if it was just part of his personality.  I was like "Well I am his wife, not an inanimate object, I fail to see the relevance!!"   People.  I swear.

I would just tell your child it didn't work out, then as they get a little older you can elaborate that they didn't treat you correctly.  My XH is a turd and I just kept my explanations to my DS mature.  I figured I would take the high road.  When he was older, like 11 or 12, I gave him some more details about why it didn't work out.  Of course, he has visitation and figured out for himself that his dad was a shit, which is way better than me telling him.