AITA for snapping on my manager bc he doesn't give a crap?

I am the assistant manager at a retail store. Our customer base is people who work in the trades and depend on us for their products. Me and this manager, ill call him M from here on out, have been working together for about 6 months now. M had left me alone for almost 3 weeks while he was away when I first started. Already giving me a bad start to this job and already setting the tone for how this workplace opporates. He frequenly leaves me alone, making me fend and struggle as i have to run the store alone. i can, but at my position, i havent been trained to do so NOR was I actually trained in my position - everything I know is bc of me training myself/learning along the way. Along with this, M takes personal calls, deals with personal things at work, leaves for unknown personal reasons, makes bets, march madness brackets etc. Anything and everything fun, personal, or pertaining to his life outside of work, he does while at work. He has also bribed me (with a coffee machine) to not say anything about him lying on his time card. I did not take him up on his offer. M and I usually get along, pretty good working relationship up until now. Just to give you an idea of the environment.

Recently we have an employee that's lazy, complains, and low-key threatened another staff member. This problem employee, ill call PM from here on out, is causing a lot of issues within our store. Everyday I'm told a different scenario that occurred from other staff members, full time and part time. I have been speaking with M about this for weeks as I, at the assistant level, cannot do much other than offer support and tell M. But M doesn't seem to care, doesn't seem to help fix anything, and isnt interested in having a team meeting. Just sits there with a blank face and nods. M likes to minimize and shove things under the rug. PM threated one of my employees after he mentioned to me that PM isn't working or being a team player. To put this in perspective - PM leaves for lunch with truck needs to be done, stares at other employees working, lies to customers, messes up orders and tries to cover his ass, is unwilling to work, has the worst attitude everyday, makes everyone else do everything etc. Ive spoken to PM about this before, but nothing seems to come from it. Yesterday, after an issues with PM, M started to minimize and make excuses for PM. I think my manager is too lazy and is too focused on getting promoted to care. I think, in his head, if he gets promoted, the problem isn't his anymore - it's mine. I snapped on M yesterday, expressing that these issues within the store need to be fixed and addressed. The smaller issues arent too crazy but the threatening and lying and attitude problems are causing the other staff members to lash out or become distressed while working. It makes it difficult for me to work and for everyone else to work. M needs to do something about this and isn't.