I know how you feel in feeling your head is a mess and actually still feeling compassion for your mate, though your intellectual side knows it's a bit insane and you must split. I am experiencing the very same. We feel sorry for them because we know their inner pain drives them to their behaviors. While it is NEVER an excuse, we somehow find a way to give compassion. If you are like me, you are very empathetic but it can often times be to our own detriment. Keep in mind that, hurt people hurt people. We love hard, but God also gave us that love to give some to ourselves too....something we seem to only freely give to others. It sucks that people have addiction and have "issues", but it doesn't mean we have to carry it too and ruin our own lives. We can give compassion and also be separate and away from those we feel compassion for.
I am so sorry you are going through this. Please take the advice of honeypig above and contact your DV center. Slashing tires and threatening you and intimidation and abuse that appears to be escalating, is frightening and you need to get out for the sake of you and absolutely your son. Also, as honey says....addiction and abuse are two separate issues, though you will find they like to hang out together on occasion! The thing is, is that even if he kicks the drinking, which he hasn't shown any desire to do it seems, he is still an abuser whose antics will only progress. Just as the drinking will.
Maybe if you can adjust your thinking to "for the sake of my son" it may make it easier? I only say this because I too struggle to extract myself from a harmful relationship because I am scared (not for my safety, just scared to stand up for myself). So maybe if you just have to blame it on leaving for your son it can be easier like, hey...I may not be strong enough to decide to leave for my own, but unfortunately my son is being impacted so I have no choice. I know it's a weird concept I am throwing out because you should never feel you need to justify leaving an abusive relationship, but maybe if you re-frame it as leaving because you OWE it to your son (which, you do), it can make things a little bit easier.
Take care of yourself as slashing tires is a pretty scary tactic that seems it could spiral badly. Sounds like you get it intellectually (like me!), but need to put the big girl panties on and cut off the relationship before things get worse....and believe me, they will. Are you getting any counseling or going to Alanon? This should be done even if and when you split for good.
*hugs*