Imagine you live in 7th century Arabia...
Imagine you are very religious and seek truth...
Imagine you run into a Muslim in Mecca who tells you about Islam...
Imagine you convert because you like the peaceful message...
But..
Imagine while time goes by you watch as Muhammad changes...
He starts asking super obedience and asks for his followers including you to start butchering those who do not believe him (most Arabs)
Imagine you thinking to yourself, Islam is no longer a religion but a political ideology being imposed by force...
Imagine you watch as Muhammad breaks his own rules and marries whoever he wants, asks the murder of whoever he wants... has sex with a kid, thousands are slaughtered because he said so... he begins stealing from others, kills anyone who is critical of his actions... you watch as your fellow muslims turn into fanatic murderers who fantasize over murdering those who see Muhammad for what he is, a warlord wanting to gain more power through violence...
Would you kill him to stop the madness?
So the question is not would I kill him if I could go back in time, it is would I kill him as a person who lived in that time already and was a witness to his change from peaceful to maniac?
Well others tried, and they died. I would try and most likely die too.
Depends though, if I lived in that time, would I not just be a product of my own society, think the way they do, behave the way they do? would I have the education that allows me to break from religion now? Can't guarantee I would.
For all I know, as a product of my society, I might have fallen for the bullshit or been one of the victims by fighting against it.
If the question is would I go back in time, then my answer is no. History is gone, as Al-Alethia said, there is no guarantee there wouldn't be another tyrant to replace him, no guarantee that the future I returned to would be better.
I think instead of wishing death on the past, we need to wish change upon the future that is coming, and just keep on seeking to educate the young, and spread that education across the world, since education inevitably leads to the death of primitive religions.