why is the eva 8 so bad

I don't think it's meant to be super strong. Double Tap can make it good sometimes in close fights. It's a good weapon for quickly thirsting people for sure though. 

I also always wonder why it had an 8 in the name and did 7 damage a pellet. It's because the recoil forms an 8 in the 9 pellets it fires. 

If Respawn ever decided to buff the damage. 8 damage per a pellet would be great for it. 

Double Tap is a great Hop-Up for it and the G7 but it can be situational. Double Tap is not always the best in every match.

I don't think it's bad at all. Purple on and it a a beast. Even without a bolt it's good. 

But yes if you go up against a good player, with good aim, good tracking  skills and say a R99 you are dead.

Most of the time i feel it's perfect. You move and dodge around, they miss and you hit, dead.

Try it out in a few games, go with something faster like R99 or R301 then Eva8 as a secondary weapon. It's a nice weapon that finish the fights thats gets up close after some armour breaking and such.