Analysis using canon from the Star vs. the Forces of Evil wiki and show details:
The question focuses on whether Mina's new Solarian army (the recruits empowered by Moon) were "always evil," given that they willingly volunteered for the Solarian Metamorphosis spell, but later sources (like TV Tropes entries) describe the new generation as realizing "too late" they weren't as hate-filled as Mina, having no control over their actions, and being happy to be freed when magic was destroyed.
Canon breakdown:
Recruitment: The new Solarians were Mewman peasants from Moon and River's forest settlement—people displaced and dissatisfied after Eclipsa's coronation legitimized monster-Mewman equality. Mina actively recruited them, scavenging old armor remnants while rallying support. Moon assisted by performing (or aiding) the Solarian Metamorphosis spell on the group, believing it would create a force to peacefully remove Eclipsa from the throne (as revealed in "Pizza Party"). The recruits voluntarily submitted to the process, motivated by prejudice against monsters, resentment over lost status/land, and right-wing nationalist sentiments (e.g., "reclaim Mewni for Mewmans").
Effects of the spell: The Solarian program (three spells: B.E.A.R., B.O.A.R., R.A.P.T.O.R.) explicitly removes fear, conscience in battle, and amplifies aggression/senses. It imbues warriors with Queen Solaria's "flame" of hatred toward monsters. This causes extreme aggression and insanity (e.g., even mild characters like Maude become raging psychopaths). Warriors swear loyalty to Solaria herself, not the caster (Moon couldn't depower them). The spell overrides normal morality, turning recruits into fanatical super-soldiers.
Were they "always evil"?
Yes, initially: They joined willingly because they already harbored strong anti-monster bigotry and supported overthrowing Eclipsa (a "monster lover"). Their baseline ideology aligned with Mina's extremism—they weren't innocent bystanders but active participants in a coup/genocidal campaign.
No, fully/completely: The spell amplified and corrupted their hatred, removing conscience and control. Many weren't as radically hate-filled as lifelong warrior Mina (exposed for centuries). Post-depowering (in "Cleaved," when magic is destroyed), most abandoned the crusade and did not follow Mina into isolation, implying relief/regret at returning to normal. A few hardliners (e.g., Manfred) stayed committed. TV Tropes' "And I Must Scream" and "With Great Power Comes Great Insanity" entries capture this nuance: the new generation realized too late the spell trapped them in uncontrollable actions, and freedom from it brought happiness for most.
Thorough conclusion: Mina's group started with evil intent (voluntary racism/nationalism), but the magic made them far worse—stripping free will and escalating to mindless aggression. They weren't "brainwashed from zero" but corrupted beyond their original level. Most weren't irredeemably evil at heart (unlike Mina, who remained unrepentant), as evidenced by the majority walking away post-magic. The trope text highlights this tragedy: willing joiners who underestimated the spell's mind-altering cost.
Basically Jekyll and Hyde type of issue.