First (((hugs))) to you and your children...I can relate to your S14 - I to have bi-polar which tends to be more angry then depressed...
I understand how he is feeling because it is exactly how I feel on any given day with my situation.
I had to literally walk away from my husband and leave the house (the night he broke the news about OW) because I knew in my heart if I stayed, he would have died that night and I wouldn't have batted an eye about it...
Going through this is heartbreaking...Throw bi-polar into the mix, and it just downright sucks...Your son (as I am sure you know) is going to feel the same things as you and the other children only those feelings will be magnified and he will act on his emotions (hence the letter, destroying property, smashing walls etc...)
Not an excuse however it makes him feel like he has a handle on the situation and his emotions...
The day after hubby broke the news to me, I cut myself - 17 times right up my arm...I haven't done that in years however no amount of medication could have stopped the downfall of emotions I felt at that time so I reverted back to how I used to cope with to much at one time and that was self-mutilation...
Please allow your son (within reason) to show his emotions, to express his anger and hurt and disappointment in his Mom....Please don't make him keep a lid on what he is feeling otherwise, it will consume him...
All of your kids should be allowed to feel and express without shame/guilt and your W should not hold any of that over them....I have family members that no matter the situation ALWAYS make it about them - Just pathetic and self-centered.
I am so so sorry for all the kids involved in any of these situations and my heart breaks for them all.
Didn't mean to write so much....I am around if you need to talk.
May All Who Seek To Take My Life Be Put To Shame And Confusion; May All Who Desire My Ruin Be Turned Back In Disgrace. ~Psalm 40:14~