It wasn't random. It was to appeal the warhawks in republican party. If you remember how it went, they were ramping up rhetorics and manufacturing consent for war with Iran, but that stopped really abruptly, like cut with knife. I don't know what was the real reason or their combination, it might be that the manufacturing consent was not working as intended, maybe ithey pushed too fast, maybe Pentagon wasn't ready, maybe because Iran shot down their most modern drone with old Soviet missile using domestically manufactured radar proving their deffensive capabilities were more significant than what Pentagon assumed. Anyway, the ramping to war stopped so suddenly that even the leading warhawk that is John Bolton was surprised and he even got sacked from WH which prompted him to publish book where he accuse Trump of being less warhawk than himself and many rep senators.

So Soleimani was assassinated to appease the warhawks and because US have to have last word.