The money quote buried deep in a Wall Street Journal article about the Soleimani assassination:
Mr. Trump, after the strike, told associates he was under pressure to deal with Gen. Soleimani from GOP senators he views as important supporters in his coming impeachment trial in the Senate, associates said.
So Trump assassinated a foreign leader, at least in part, for political gain. Specifically, he wanted to avoid being removed from office for withholding U.S. aid to an ally for political gain.