So, if I go with dashing scoundrel lego, wouldn’t I want some some high mastery? I know haste is needed a bit, but mastery directly increases damage and with assassination being poison based mostly, wouldn’t I want a higher mastery than say haste? I solo a lot, but I do raid casually every so often.
So haste is important, because it increases the rate at which your poisons/bleeds tick. Giving your more damage per bleed/poison, and due to your passive Venomous Wounds, more energy which turns into more fillers which turns into more finishers… It also increases attack speed, and this is where your instant poison comes into play
Crit is important because it gives you free combo points which leads to more finishers
Mastery, while it does increase overall damage, the increase you get from other 2 stats outweighs the benefits from Mastery.
Given that, when I play Sin, I go heavy Crit/Mastery with haste just behind Crit and I keep up just fine 
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So should I stack crit/haste since those two are so important? I’m not sure if this is average, but I’m doing about 3.7k at ilvl 237ish. I feel like its below and I’m trying to improve.
When in doubt, sim it out.
https://www.raidbots.com/simbot
https://www.askmrrobot.com/wow/simulator
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I mostly play sub but if I want a laid back night, I play sin. My gear is mostly optimal for sub. 35% crit, 20% haste, 25% mastery(trying to trade that out for verse) and 15% verse.
With that being said, sin doesn’t feel bad at 20% haste but it would perform a lot better with 25+% haste.
The first reply covers the reasons pretty well. Verse > mastery because it effects your auto attacks also.