So telling someone some things makes you responsible for how they act on that information? What secret did Unser know that would have changed Jax's life?
Dude wasn't perfect, but he was for sure not guilty enough to be murdered in cold blood.
Last but not least, no one is saying that Milo would have ended up in a padded room, but watching a person die fucks a person up. You never forget it. Watching someone splatter right in front of you would definitely fuck a person up unless they were a psychopath.
You're ignoring all the context in Unser's action. He knows Gemma and Tara. He knows the history between the two and the many altercations between them (let's just name the fact that Tara would go as far as to fake abortion caused by Gemma hitting her). He knows Gemma's temper. He knows how she holds Jax above else to the degree she raises hell on the idea of him moving out of Charming with Tara. So with all that perfectly known to everybody including Unser, he still comes running with hot, second-handed, uncorroborated information that TARA MADE A DEAL THAT SENDS JAX TO JAIL.
Even Chucky understands the implications of that. Unser has painted a target on Tara's back, set the fire underneath Gemma's ass and gave her all the ammo she needs to go confront Tarra and do something rash. Why? because he's so pathetically love stricken that he would go and tell her valuable and critical info despite knowing very well what it's possible to cause.
And you know, then you add up all the years where he betrayed his job and illegally helped SOA out. 'Innocent' he is not.
As far as i remember Unser knew of Gemma's consent of killing JT. If Unser wasn't her lackey then he might have ventured to tell Jax that as early as possible which could have circumvent all the shit that went down over the years.
Milo would have affected but it's not something i would call 'life ruining' and frankly not something i would care about for such an insignificant character. Jax's 'rap sheet' is long enough that it's incredibly unnecessary to add to it what impact Jax's suicide would have on a character than 5 minutes of screen time in total.
It's also funny because of Jax's involvement in Eagle being severely shot (and her partner killed) but her quickly absolving him and holding no hard feelings.