so, slate warrior has 2 effects. the first one i know to be a flip effect, and he gets 500 atk and def points. the 2nd, however, i'm not so sure about. is it graveyard? or went hes sent, after he's flipped? also, if skill drain is on the field, does his 2nd effect still work? AND, if slate's was activated first, THEN skill drain, would he drop back down?

The secondary effect is NOT part of the FLIP: effect. It's a completely separate effect that takes place immediately after it is destroyed as a result of battle. It does NOT matter whether it goes to the Graveyard or removed from play.

It is not a Graveyard activating effect.

Like Apprentice Magician and D. D. Assailant and such, it activates while considered "face-up on the field" and activates during the Damage Step and it will activate if destroyed in battle no matter whos side of the field it is on, so if you destroy it in battle while it's under the opponent's control, your own monster will end up losing 500 ATK and DEF.

Skill Drain WILL stop its effect from taking place since it is negated. And, obviously, stuff like Ha Des, Dark Balter the Terrible, etc will negate its effect if they destroy it in battle.