I don't formally invite people to my flat the majority of the time. Perhaps a few of us go from a bar back to mine for a couple of drinks before going somewhere else.

Formal or not it's still an invitation and once they are at your house you are now technically responsible for them until they get home or go to someone elses house, at least in Canada anyways.

I don't really see why I should have to offer them somewhere to sleep, there really isn't much space anyway.

You should never have invited them over if you cannot provide someone a safe place to sleep if they are too messed up to get themselves home. Drinks have a way of catching up to people and you can go from being okay drunk to falling down drunk really really fast.

why should I let someone I'm only acquainted with stay, they could murder everyone in their sleep for all I know

If you are worried about someone murdering everyone in their sleep or otherwise why were they invited at all?

am I also liable for what nutters do inside my house?

You are especially responsible for what people do IN your house, if someone goes into your bathroom and ODs and dies you could be sued by their family or whomever is overseeing their estate.

If someone bails while I'm in a different room I can't go chasing them down the street to make sure they aren't going to try to drive, I don't even know if they have a car. Hell, if they want to leave, I can't physically prevent them without assaulting them.

Its not a black and white situation. You can't really lay your hands on them to stop them but you can take their keys away from them. If they continue to try to leave or whatever that is when you call the police and inform them what has happened and then they deal with the drunk person and you have shown that you did everything in your power to prevent them from going.

Why am I responsible for anyone just because they're on my land?

Well they are on your land. If they hurt themselves on your land it was something on your land that hurt them. Furthermore, this is more about being responsible for an intoxicated person after they left your supervision. In the eyes of the law/court you knowingly let them leave while they were too intoxicated to take care of themselves so their damages become your responsibility.

if someone locks themselves in my bathroom without any indication something is up and then injects themselves with a deadly cocktail of drugs it's totally unreasonable to expect me to know that.

This is something that could be argued in court for sure but you would have to prove that there was nothing you could have done to prevent it from happening.

I thought Canada had a better legal system than the US, more grounded in common sense like most European systems, but apparently not :/

The Canadian legal system just does things a little differently than the US. It's better in some ways but way worse in others.