Yes to all your questions.
I distinguish between hurt and harm. Have I hurt someone? Oh yes. Have I harmed someone? Yes. Have I done it on purpose knowing full well what I was doing? Yes. Was consent always part of the equation? No. But I think those question would be answered by most people in the same way and doesn't make me in any way special. Especially when we are talking about teenagers.
Non consensual harm would include for me making a break up as painful as possible.
Have I lost control? Yes. Most almost always it involved me being cornered, and then it was a measure of defense. A shield, emotionally, and a method of assertion through mental or physical violence. I would find myself in that mindspace. What I would later call topspace, or "baring my fangs". Being aware of it makes all the difference. I have found behavoir patterns of mine that cause hurt or harm and ended them. This has gone so far that I have even found myself refusing to say certain things if I think they assert my will over someone else without their consent.
Where I think it comes from. The primal entity of the id given will and shape. It's deep enough to be part of the core where causality breaks down. What caused it, nobody knows for sure. Lots of theories, very little data that shows a common thread. It's kind of like asking someone what caused them to become gay.
A desire for authenticity is a definite trigger for my more sadistic tendencies, though.
Edit:
@
Urtehnoes
There isn't a problem anymore.