Believing Trump is a Russian controlled agent is, acknowledging that Trump is deeply corrupt and bribed by foreign dictatorship is not (not necessarily Russia, but in general). The bribing is happening out in the open for everyone to see. Also just judge Trump by his actions. US support for Ukraine has been scaled down dramatically and largely has been taken over by Europe. Not only that Trump tried to bully Europe into stopping the support as well. Of the stuff that is supposedly still going to Ukraine, much is actually in question. At the same time Trump pushed repeatedly for a "peace deal" that was almost word to word what Russia demanded from Ukraine.

Why Trump is doing that doesn't really matter, there are plenty of reasons explaining that without him being a Russian agent.