Shylock is a very complex character. He is harsh and mean and demanding. He does not endear himself to the community asserting that he will get his "pound of flesh" from Antonio and that he hates him "for he is a Christian" (I.iii.42). Getting his "pound of flesh" will become a main focus of the play and the figurative and literal meanings of this become very important.
In assessing whether Shylock is a villain for villain's sake or perhaps a victim of circumstance and then deserving of our pity, it is necessary to consider how he has "suffered" at the hands of the Christians. The Jews are well-renowned for their vision of justice and retribution and Shylock asks the audience to consider how he has "learnt" from the Christians:
The villany you teach me I will execute; and it shall go hard but I will better the instruction.( III.i.62-63)
Effectively, Shylock is challenging the stereotype thrust upon him by a hypocritical Christian community. Christians of the time would have been forbidden from practising money-lending due to it being "unbiblical" but Christians allowed Jews to be the money-lenders as
they were going to hell anyway!
Shylock sees Antonio as a villain. He does not interpret Antonio's actions as kindness when he loans to friends without charging any interest. Shylock sees this as robbing him of business.
Antonio attacks Shylock's honor and dignity and any man would retaliate in an attempt to regain some measure of respect. Therefore, we should feel pity for Shylock. His religion is his backbone and the strength by which he lives. The Christians have stripped him and even effectively "stolen" his daughter and yet they expect him to behave any differently.
As far as Shylock is concerned, Antonio is not honorable and Shylock's actions become circumstantial. It would make no sense to Shylock to demand anything less from someone else than he would demand of himself. He does not see one crime as any less than the next. Hence, whether it seems too harsh would escape his understanding and culture. Antonio must pay - with his life!
As for the "Christian" actions that follow; it is hardly compassionate and christian to destroy a man's livelihood, force him to renounce his religion, give his money to the man who "stole" his daughter and not retaliate.