Whereas I would agree that repentance is a necessary part of the sinner's response, these two verses do not refer to individual repentance for individual salvation from sin. Rather, they refer to salvation from being killed by the ruling government.
Look at the context: Jesus had just spent a chapter speaking to his disciples about fearing God who can destroy both body and soul more than fearing human powers that can only destroy the body, and about acknowledging the Son of Man, even when they're put on trial for the same. He warned them to not worry about storing up treasures for themselves, but about being rich toward God. He continued in this vein, telling them not to worry about clothing, or food, or other earthly considerations, but rather to seek God's Kingdom. He warned that he would bring division, setting family member against family member, finishing up with a warning that they needed to recognize the signs of what's coming, and to settle up with their adversaries rather than asserting their own rights.
In this context,
...some people came and reported to Him about the Galileans whose blood Pilate had mixed with their sacrifices.and his response:
...unless you change your minds, you will all perish as well!And then he mentions:
...those 18 that the tower in Siloam fell on and killed....and once again repeats:
unless you repent, you will all perish as well!Notice the phrase that mentions a tower falling on people and killing them. We see a similar phrase in 1 Kings 20:30. The context is a battle; the losers run away into a walled city for refuge, and their enemies come up to the city. The result of the battle is that:
The ones who remained fled into the city of Aphek, and the wall fell on those 27,000 remaining men.which indicates to me that the enemies outside the city knocked down the defensive wall, allowing them to get into the city and kill the remaining 27K soldiers.
I suspect that the tower in Siloam was a hold-out where rebels had taken refuge, and the Roman government had resorted to destroying the tower itself in order to destroy the uprising.
I believe Jesus is telling his disciples not to get involved in political scuffles they can't win; they have a higher calling to put their resources into fighting for the Kingdom of God, not for the Kingdom of Israel. If they don't change their minds about their political leanings, they will wind up being crushed by the government just like these other rebels. He finished off by telling a parable which essentially said that they were due to be cut down, but that God would provide them just a little more time to bear fruit which could save them.
So whereas we're told elsewhere that God-seeking people must repent "from sin", I believe this passage does not address that issue at all, but rather that Jesus is speaking here specifically to Jewish patriots about repenting from "earthly politics".