Q: If the 1000s of Jews in Uman were masaquered why did they require a tikun?
A: Although the reward dying al kiddush Hashem is extraordinarily great, it does not necessarily mean that a neshamah had fixed everything it needed to. For even if a neshamah fixed what it needed for itself, it may not have finished the work it needed to for other neshamot that were connected to it.
Additionally, even if the kedoshim were fully ready to go, it seems that there were other neshamot who had died earlier that needed a tikkun.
Finally, it is possible that someone who is being killed al kiddush Hashem can spoil the mitzvah, Rachmana litzlan, and forfeit the tikkunim he might have had.
Hope this helped.