Oracle Text

Wither (This deals damage to creatures in the form of -1/-1 counters.)

2R, Q: Hateflayer deals damage equal to its power to any target. (Q is the untap symbol.)

Card Rulings

8/1/2008 If the permanent is already untapped, you can’t activate its ability. That’s because you can’t pay the “Untap this permanent” cost.
8/1/2008 The “summoning sickness” rule applies to . If a creature with an ability hasn’t been under your control since your most recent turn began, you can’t activate that ability. Ignore this rule if the creature also has haste.
8/1/2008 When you activate an ability, you untap the creature with that ability as a cost. The untap can’t be responded to. (The actual ability can be responded to, of course.)
8/1/2008 Wither applies to any damage Hateflayer deals to a creature, which includes both its combat damage and damage from its activated ability.