The amount of misinformation surrounding hateful strike is amazing, even after 30+ months of original Naxx release.
There are only two viable targets for hateful strike, and these are the two people 2nd and 3rd on his aggro list within melee range. Hateful strike will hit the highest HP person out of those 2. 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 2 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.
If you look at it as steps in evaluation, assuming that your 3 tanks are the top 3 on the aggro table in melee range, then the other people in the raid might as well not exist because its already eliminated them as potential targets.
Patchwerk will use this ability at a 1 second frequency on the target in melee range with the highest HP who is also one of the top three on Patchwerk's aggro list. Cannot be used on the main tank unless there are no other targets. Does 79,000 to 81,000 raw physical damage which can mitigated by armour and parried/dodged. This is about 23,000 damage on a partially Naxxramas geared tank. This ability will add threat to the three most threatening on Patchwerk's aggro list.