Who else was a "disillusioned useful idiot" at the time that the Soviets expelled unconditionally, along with his/her spouse? There are no other such cases. The disappointed Western idiots (and there were a few of them) just stayed put in Moscow. The Soviets liked to kill people, and that was sufficient interest for them to kill Kennedy (if in fact they did.)
I bet you won’t name a sufficient number of Western leftist idiots forcibly stayed put in Moscow. Most of them were failed intelligence assets and had zero interest to get back home to be prosecuted.
They haven’t even killed their own dissidents after 1956, mostly deporting them abroad (Alex Solzhenintsyn, Joseph Brodsky, Nathan Sharansky to name just some of them).
There was no Soviet interest to kill Kennedy. Bay of Pigs disaster is not a sufficient reason because it did favour to both Castro and Soviet diplomacy ridiculing United States. It was also effectively used by Castro to purge his opponents.
But even if they really wanted to kill Kennedy it was beyond stupid to use Oswald who had obvious Soviet ties. In 1963 KGB had numerous uncompromised assets in US who could do the job just fine