where merchants were taking advantage of poor widows etc

Where are you getting that from? The bible says nothing of the sort. It says "And He entered the temple area and began to drive out those who were selling and buying on the temple grounds" Both selling and buying. Jesus cast out the poor old widows who just wanted to worship the way God told them to. The vendors were selling offerings that people could burn as part of their worship and animals to be sacrificed. They were providing goods that were necessary for worship at the temple. It is not at all clear what Jesus was complaining about.

If we take it literally, I see two options:

1. He did not like that there were burnt offerings and animal sacrifices, but Jesus alludes to the temple being a house of prayer, which is a reference to Isaiah 56:7 "their burnt offerings and their sacrifices shall be accepted upon my altar; for my house shall be called a house of prayer for all people." The burnt offerings and animal sacrifices were intended by God.
2. He just didn't like commerce within the temple. He complains about the temple being turned into a den of thieves, not about there being thieves at all. Which means as long as the den of thieves was outside the temple Jesus would have been fine with it.

As far as I see, neither of these justify assault.