If Zilortha is dealt 2 damage during the turn, can it be killed with a -1/-1 effect?
First of all, contrary to what Arena shows you, normal damage doesn't reduce toughness. Zilortha still has 3 toughness even when it has taken 2 damage, so reducing its toughness to 2 won't do anything thanks to its ability. Giving it -3/-3 would kill it, however, since dying to 0 toughness is a different state-based action than dying to lethal damage.
Zilortha's ability causes you to compare marked damage to a creature's power rather than its toughness, so Zilortha would die if it had 7 damage marked on it. (Creatures with 0 power won't die unless they have at least 1 damage marked on them, though. Lethal damage is only checked if there is some amount of damage marked on a creature.)
