Ooooh, absolutely yes to both. Sneak attack as 12HD rogue, whatever that works out to be. Could maybe also find justification for poison use here: "No trick is too cruel, no tactic too immoral, no job too unsavory for them to undertake." But it doesn't quite seem their style.
You know, I wonder if "If they reveal their true form they can cause fear in creatures of 3 Hit Dice or less." is more like a fear aura based on a fear or cause fear spell rather than frightful presence. But it's not a big deal.
I was thinking more an assassin-style death attack than a sneak attack, but we need the latter to qualify for the former.
Death Attack (Ex): If a slayer genie studies his opponent for 3 rounds and then makes a sneak attack with a melee weapon that successfully deals damage, the sneak attack has the additional effect of possibly either paralyzing or killing the target (genie's choice). While studying the victim, the slayer genie can undertake other actions so long as his attention stays focused on the target and the target does not detect the slayer genie or recognize the genie as an enemy. If the victim of such an attack fails a Fortitude save (DC 18) against the kill effect, she dies. If the saving throw fails against the paralysis effect, the victim is rendered helpless and unable to act for 1d6 rounds plus 1 round per level of the slayer genie. If the victim’s saving throw succeeds, the attack is just a normal sneak attack. Once the slayer genie has completed the 3 rounds of study, he must make the death attack within the next 3 rounds. The save DC is Intelligence-based.
If a death attack is attempted and fails (the victim makes her save) or if the slayer genie does not launch the attack within 3 rounds of completing the study, 3 new rounds of study are required before he can attempt another death attack.
Since slayer genies are all about the slaying, should we keep the paralysis option? I suppose their master could order them to capture someone alive, but I'm not sure they do that!
Non-paralysis version:
Death Attack (Ex): If a slayer genie studies an opponent for 3 rounds and then makes a melee sneak attack that successfully deals damage, the opponent must succeed at a DC 18 Fortitude save (DC 18) or die. If the victim’s saving throw succeeds, the attack is just a normal sneak attack. The save DC is Intelligence-based.
The slayer genie can undertake other actions while studying the victim, so long as his attention stays focused on the target and the target does not detect the slayer genie or recognize the genie as an enemy. Once the slayer genie has completed the 3 rounds of study, he must make the death attack within the next 3 rounds. If a death attack is attempted and fails (the victim makes her save), or if the slayer genie does not launch the attack within 3 rounds of completing the study, 3 new rounds of study are required before he can attempt another death attack.