What is the difference between constructive criticism and abuse?

One is helpful.
The other one is patronising and insulting.

Intent ...

Your perception of the person giving said constructive criticism.
If you don't have a good rapport with your immediate senior, it's always going to be hard work and you'll never feel comfortable in each others company.

When the giver of criticism can't receive it in equal measure.

When there's nothing you could do to please the critic that he won't just find something else to criticise.

"faggot"
"Could do with some improving in this area"
ʕ ᵔᴥᵔʔ

way its said. words matter

Perception.

Maybe a baseball bat to the head? Something like that.