It's called picking, and it's the most important concept in Construct programming.
Use events to pick instances of slime1 object.
In your example "On collision" event picks 1 instance of slime and 1 instance of bullet. Inside this event you can only control these instances.
There are lots of events that can pick instances, say "Slime1 is visible" event will pick all visible instances.
"System -> For each slime1" event will loop through all slime1 instances, picking one instance at a time.
If you don't pick any instances in the condition (left) part of the event, than everything you do in the action (right) part will affect all instances!
And by the way, read my comment above about families. You should add all your bullets to a family, and remove duplicate "On collision" events.