There are only three viable targets for hateful strike, and these are the three people 2nd, 3rd, and 4th on his aggro list within melee range. Hateful strike will hit the highest HP person out of those 3. Additionally, hateful strike will add threat to the target it hits, which helps keep these tanks on that hateful strike list. This is the reason where if your 3 hateful tanks are low hp, he would still hateful one of them and kill them, disregarding the fact that there were melee dps with higher HP.

Typically, the threat added by getting hit by a HS is more than enough to maintain these tanks on the hateful list, however there were occasions where tanks dropped off the list due melee dps pushing more threat than what HS gave your offtanks, but that was only seen during times where you'd run the old Naxx with level 70 people.

Sinking your melee into the slime doesn't help anyone, it's just stupid and you should quit doing it.

Two corrections:

1) It's only the #2/#3 in 25-man, and #2 in 10-man.
2) When he HSes it actually adds threat to EVERYONE currently in the top 3 (or 2 for 10-man) on the threat-list. This is 100% transparent now. Just watch Omen or other threat-meter, during the fight. Every time he does an HS, everyone at the top of the list will see their threat jump upwards.