But doesnt that just show us that Rex is only free to voice his actual opinions once he knows he is getting fired anyway?
Indeed it does.
I am clarifying that he was not fired for saying that, which would imply Trump is firing anti-russian advisers as this is what it looks like to an average person.
US so far did a reasonable thing, which is voice support for their ally. Also Tillerson said a very reasonable statement that it is probably the Russian government who is behind this.
Edit: Apparently Trump is citing Iran disagreements with Tillerson as the reason for the sacking.