Alex isn't going to die in his own fire. He's gotten away with a very public arson, possibly two, at this point. In the case of Tim's house he was clearly prowling around for a fight while setting the place to light up, yet left cleanly without so much as an injury.
And we're giving him a day of prep. Alex doesn't just burn things down, he clearly sets traps and does it strategically, and he gave himself an exit plan to get away with his crimes but also minimize the danger to himself.
Plus Alex doesn't fight head on, he's a snake who sets up ambushes and tricks people, an abandoned hospital with time to plan? Alex's stealth will let him get the place lockeddown and set up traps and ambush points, as well as tuck away evidence like the empty tanks.
Gas might have a strong smell, but SBA means the fighters can't really just leave. And the hospital in general will reek when he's done with it, avoiding parts of it that will be more dangerous when Alex decides to light up is going to be much harder for Fleck (even WORSE for Fleck should he walk around in it), but Alex will in all likelyhood have his own safe path prepared. He had an exit strategy and a backup plan when trying to burn Tim alive, he took out his cast and Jay by watching, planning, and waiting until ambush time, Alex's experience, stealth, observation abilities, and patience mixed with prep is a major boon. He's insane, not stupid.
Fleck's obviously physically superior, and might be better with his own firearm, but his pain tolerance and durability won't hold up well to flames (not to mention how distracting being burned alive probably is) and while his tolerance might allow him to power through a gunshot wound, piercing strength of a bullet will still put him down with a well placed shot without giving him a chance. Not to mention his best victory condition is to catch his much stealthier opponent in an environment that plays to his favor rather than getting caught in a fire trap or ambush himself.
We're giving an experienced and stealthy planner, manipulator, arsonist, and murderer a whole day of prep and tanks of gasoline against an opponent who might be stronger and tankier but doesn't have the same stealth and planning skill. I think the votes are entirely valid, Alex is at home here, he can quickly turn this into hell for Arthur.