For the left Melenchon is basically a destructive troll and for the same left his Presidency would be even more disastrous than Hollande's to PS. I'd certainly take him over Le Pen or Fillon, but I'm not sure I wouldn't pick Macron instead if the choice boiled down to the two.
Mélenchon is a terrible person, but at least his program isn't a bunch of vague yet distinctly harmful neoliberal platitudes. He'd definitely do less harm in terms of policy, at least.
"This candidate is a worse person than that candidate but has less harmful policies" isn't whataboutery, it's a normal political calculation.
