Invading Iraq was done after an enormous amount of public debate, with the broad approval of Congress, the UN, etc. It may not have ever been the right decision but it was unambiguously "okay" from a following the rules in letter and spirit sense. These assassinations are being done in secret by a president who claims he doesn't have to explain himself to anyone.
There isn't a ton of upside in invading a country, compared to the amount of effort and oversight involved. There are easier ways to funnel money to the Military Industrial Complex or distract from domestic political shame. The only real upside is ideological, either rightly or wrongly you think it's a good idea for whatever faction you align yourself with.
But assassination of the local population is so very useful. It's a Dremel that a bad leader can use as a half-ass solution to any problem. For the kind of person who would aspire to be President, the logic of assassination is so clear that you don't even have to be paranoid: "That person is an inconvenience to me. I have the power to kill him. Kill him."