You have the option to committ genocide about six times in the
Mass Effect series:
1. On Noveria, in ME1, where you can spare or kill the last remaining Rachni queen. You have the chance to save her again in ME3, where the choice is between her and Grunt's Platoon.
2. During Legion's mission in ME2, where you can destroy or turn the renegade Geth.
3. The final ME2 DLC Essentially has you destroy the Batarians' collective homeworld. The only way to avoid this is to not do the DLC.
4. You essentially destroy the Collectors entirely in ME2. The only ones left are remnants used by some of the Reapers and the renegade Levianthan faction.
5. During Wrex's segment in ME3, where you can help save the Krogan or thwart their cure.
6. Your actions result in the deaths of either the Geth, Quarians, both, or neither.
In all of the above, with the exception of #3-4 where the choice is made for you, mercy is the best choice:
1. Sparing the Queen results in her being an ally in ME3 once you do her subquest, provided you spared her the first time as well, resulting in higher levels overall and some interesting fluff from helping her out.
2. Saving the Geth means more allies against the Reapers later. See #6.
5. The Krogan have been DOUBLE GENOCIDED if you decide to thwart the cure. They've been losing numbers for years and it's been getting worse because of the Genophage. Fucking up the cure also betrays
everything Mordin Solus' character arc worked towards, you fucking monster.
6. This is solely on you. Genocide the Quarians, Geth survive (and with greater numbers if you converted them in #2). Spare the Geth, the Quarians die. Try to broker peace without the stats, they kill each other. Pull it off and you cause one of the most emotional, moving scenes in ME3 and add two of the largest spaceborne armadas in the galaxy to your cause.
I'll bet Wu doesn't even headbutt.