Fun facts: the only thing that was noticed in eugene's post mortem toxicology report was cannabis (though there were unidentified & undissolved pills in his stomach) and eugene DID previously interact with the victim (ronnie poppo who luckily survived the incident) as part of an outreach program to the homeless
given eugene's violent & criminal past (frequent domestic abuse that led to divorce alongside thrashing a house, beating his mother and threatening to kill her) it's likely he had some type mental illness that leads to aggressive behavior and the miami sun baking him in the car led to the drugs he took being absorbed while he was dehydrated (eg; in a more undiluted state) and not quite all there upstairs to begin with from heatstroke (which can manifest as confusion, aggression, paranoia - which is corroborated with ronnie saying the attack happened after eugene randomly accused him of stealing his bible after going on tangents about the apocalypse)
...which could also explain why he ditched his clothes