Isnīt it obvious that Trunks suffers from some kind of PTSD? Seeing the appearance of Gokuīs reminded him of Black Goku, who just killed his mother and love interest. Which, btw, was super-gruesome. The way you could see future Bulma fade away as her body was evaporated by Black Gokuīs energy. On the other hand, Mayīs death was super pointless. "Iīll buy you some time, Trunks, go!" - Trunks keeps lying on the ground, watches as May is killed. lol.