Teachable moment for 19 year old... AITAH?

Am I the asshole?

Back story:

We have 4 children (19m,11m,10m,5f) some of which have special needs (19m has high functioning autism), (11m has social emotional autism also high functioning) (10m has adhd / early signs of depression), (5f none that we know of but does have "last child syndrome" (everything is hers, gets her way, still isn't fully potty trained, "does what she wants")...blended family since 2017 (19m hers, 11+10m mine, 5f together)... don't ever really think of it as "hers or mine" until situations like this happen...

Me, Dad (tries and definitely not perfect, been trying hard for 2.5 years to be better and hold himself and others in family accountable for their actions, battles depression and own "demons" since own father died in 2020, can't find a good fit career wise as engineer in manufacturing) Mom (gives of herself when doesn't have it left to give, burns candle at both ends in nursing 3rd shift and in school to be physicians assistant, adhd & depression herself. 85 year old Dad is "dying"...(noticeable changes in health and quality of life for him) All other family support is MIA....

Story:

Out shopping for Easter with wife, kids at home with big brother...everything going good-ish...*10m* is slacking on household chores via constant distractions. Multiple calls from oldest to complain about trivial issues (normally happens). Get home and sit with *10m* to ask "What's with the attitude towards big bro?" Denies attitude...Me Dad checks cameras (*11m* is somewhat flight risk and also pets and lots of "chaos" that needs supervision even from afar at times, doesn't check unless needed most of time "sherlock holmes" style) I find out that Big brother unintentionally lied about little brothers' attitude and was actually him who had the stick up his butt most of the afternoon. Confronting big bro happens... told him that he was in the wrong 2 times in quick succession while explaining who actually got it wrong (trying to use it as a teaching moment) 19m denies both times and tries to back out and twist words of what he said instead of apologizing for almost getting brother in trouble (the whole accountability thing from earlier)...i get upset for being made into a liar via my oldest and I lose my cool and yell at him for not fessing up to his actions and being defensive. I tell him to apologize at this point to his brother and to "man up" for his mistakes and actions which gets me yelled at by the wife. Oldest through tears (can turn on waterworks instantly when caught up in his own bs) says sorry and I yell for him to go to his room. On the way to room he begins to self harm and act out (banging head and fist on wall, head in hands leaving scratch marks on his cheek)... this is semi-new behavior for him... has only started since becoming an adult and being fully held accountable for his actions and/or inaction by me (Mom, Gma, and Bio-dad treat him with kid gloves)

Now!!! Am I the asshole for trying to hold him to an adult standard and trying to have it be a teachable moment? I'm sure my temper could have been more in check (it's waaaay better than it used to be tbh) This and a few other instances of me trying to prepare him for "real life" have blown up in my face like Acme dynamite because of the kid gloves and hand holding from others...Keep in mind as well he has a job, in college (so not a box of rocks up there), and has all the support in the world to help him succeed if he just asks (and is offered most of the time even when he doesn't ask)... what do you think?