I'm looking at this from the outside, as a foreigner, but my impression is that pro-Trump people don't really think much about what's going on between Russia and Ukraine. They voted for Trump, and they are doubling down on their choice regardless of what he says or does.
In terms of big picture and how this evolved, my impression is that Trump is using a strategy previously used by Hitler. That is, he needed to unify people to follow and support him regardless of what he did. What better way to achieve this than to convince people that they are treated unfairly, cheated, and victimized by someone. Trump started by announcing that immigrants are criminals and murderers. They are the "bad guy" that is victimizing the USA people. Hitler named Jewish people as the "bad guy" After he was elected, Trump expanded that 'bad guy' image to include Panama (they stole the canal), Denmark (won't sell Greenland), Canada (replace "trade deficit" with "subsidy"), the EU (formed to harm the USA), and Ukraine (grifting for war supplies). Each of these "bad guys" are, according to Trump: so mean, so unfair, so nasty to Trump's followers (victims).
People who elected Trump were gradually convinced, through daily repetition of false statements from Trump, that they are victims of immigrants, Canadians, Ukrainian and many other countries. Today, Trump's supporters are unified in their belief that they are victims who are justified in lashing out at all those people/countries that Trump identified as harming the USA. The victim mentality is so deeply rooted that Trump and his followers tell Canadians that they should not retaliate when Trump enacts Economic War against Canada on Tuesday. They believe that Trump is justified in declaring war, that Canadians have harmed the USA, and that any reaction is an invitation to escalate the war. They believe that Canada should do what Trump says because, according to them, Canada is stealing from the USA. They do not accept that Trump has no authority in sovereign countries like Ukraine, Greenland, Canada, etc., or that Trump is concocting facts.
Similarly with Ukraine, Trump's followers/victims look at the $6.6 billion spent (no strings attached) by the USA and the $6 billion spent by the EU (to protect democracy), and conclude that Ukraine is cheating the USA. They watched Vance and Trump deteriorate to infantile shouting-match tantrums with Zelenskyy and then blame Zelenskyy (echoing Russia) - with the usual name-calling, insults and declarations that the man who had the tantrum was justified since, in their view, Trump is the victim defending a country full of victims. Trump's followers/victims believe that Ukraine is a 'cheat' that deserves mass murder from Russian troops because they accepted USA money to defend democracy.