My take on them scrapping the sniper plan so quickly was not only because of the Eugene factor, but also because of their prior impatience (especially Rosita) and the bonding they did in the warehouse right before. I mean, the main reason they went after Negan on their own was because they didn't want to wait for Rick and the rest of the group to do what they needed to do. They wanted immediate satisfaction, and they were willing to give up their lives to get it. Rosita was planning on going in, guns blazing right up until they were almost there and Sasha convinced her otherwise. Combine that with their bonding moment when they said they had each other's back no matter what, and I didn't find it all that surprising that they scrapped the sniper plan.
Yeah. I still think it was dumb though. They had the advantage of an easy shot from distance with the element of surprise. The two of them, and eventually just the one of them, going in guns blazing kills any advantage they had.