My understanding is that Soleimani was responsible for the deployment of a type of anti-vehicle mine in Iraq that killed hundreds to perhaps in the low thousands of US soldiers. A lot of the military brass wanted him dead for personal reasons and Trump was too weak to resist their urging he be taken out. Trump may have avoided immediate consequences because Soleimani was more useful as a martyr to the Iranian leadership at this point. In the long run it fueled another generation's anti-US animus.