It's a pretty simple line for me. I truly start to hate someone when their bad actions change from being annoying/distasteful to outright abusing people and causing them significant pain.
I tend to throw around the word "hate" a lot for really petty things, but I could probably count on one hand the people that I truly despise. Like despise as in would probably light on fire.
Let's just say tu have to do something pretty bad for me to hate you. I start to hate someone if they did one truly awful thing o if they are persistently awful to me.
Example: I dislike that kid who was an culo to me in third grade. I hate the kid who treated me like shit every other día for about three years.