RiotNrrd:

qwertz:If there's no victim somebody doesn't need to mark a victim (=accusative). Is that right?

That's a violent, but basically accurate, way to think of it.

Sometimes the sentence is all about the attacker, and no one else.

The attacker is angry.
The attacker is becoming red.
The attacker seems agitated.

None of these is about anybody but the attacker, so no crime has been committed (yet), and so there's nothing and no one to accuse him about.

Once you add the victim, well...

The attacker shot the victim.
The attacker stuffed the victim in the trunk of his car.
The attacker hid the victim's body in the abandoned well.

Now there's some accusations to be made. And so you mark the thing that the accusations are about (i.e., the victim).

Not a grammatically pure way of putting it, of course. More just a fun way to try and remember how it works.