Well Were they justified in killing European settlers after the fact?
It totally depends on the actual facts. Some facts state the mass disease was not done on purpose. So no it would not be justified. Some facts state that a commander wrote a letter to infect them with small pox infested blankets but with no evidence he carried it out at all and also them knowing they had to infect them within 24hrs pretty slim But NOT impossible but with o evidence, again no it would not be justified.
BUT if he did know and did do it on purpose. Then yes it would be justified. And I'm sure there are other facts that it would be justified.
BUT it was LONG ago why dwell on the past you or anyone alive did not experience.
Remember your ancestors for the good not the bad. Remember what you know for a fact pass it down to your children but don't teach them hatred because of a past you had nothing to do with. And never allow it to happen again, which the chance of that is zero.:)