Chameleon Circuit – Regenerate Me lyrics

This is about the doctor.
He travels throughout time and space in his tardis, and he tries to fight against evil everywhere, and no one can defeat him. Whenever someone or something almost kills him, he regenerates. I think the use of the word "exterminate" is a reference to the daleks, who are the doctor's oldest enemies and like to say that word a lot.
The next few lines after the first two "regenerate me"s are about how the doctor travels alone in his tardis (he does have companions, but none of them are with him forever). They are also about how the doctor is the last of the time lords, after they were destroyed in the time war.
He tries to fight for justice everywhere and makes sure that wherever he goes, things are fair and just. "i am your last chance to repent," refers to how he tries to change his enemies ways by peaceful negotiation before going to violence.
The "ten personas" refers to the 10 different incarnations of the doctor (before matt smith). The rest of the song talks about how the doctor tries to protect humans to the best of his abilities, and how him and his tardis are woven into myths and cultures around the world as a protector and healer. The last line refers to the fact that he regenerates, so his physical appearance changes, but his motives never do.