Complex sentence: They were Kufa's people who invited Husayn to themselves to be their Imam, so they saw themselves responsible for the tragedy of Karbala and tried to compensate for it by throwing themselves into the struggle to obtain vengeance for Husayn's blood.
Split into simpler sentences: They were Kufa's people who invited Husayn to themselves to be their Imam, so they saw themselves responsible for the tragedy of Karbala. They felt remorse and tried to compensate for it by throwing themselves into the struggle to obtain vengeance for Husayn's blood.