It doesn't matter if your passengers kill anyone, I did it on Uplift Reserve, gave my Splaser to a marine and he did plenty of damage.
Most likely you didn't get the achievement because you honked the horn. The first time I tried I didn't get it because I honked.
Here's a tip (assuming you did it on Uplift): After the first major area with the 2 Wraiths, you'll come across a beach with a road overlooking it that you have to get on. The road has a lot of Shades on it that will shred your vehicle. Before you go onto the beach there's a bunch of giant rocks that you can jump onto (where the banshees first show up) and then onto the road to bypass that area entirely (you need to crouch-jump). There's a ghost nearby and a few grunts that you can just melee. And if you see anymore Shades, just run them over.
[Edited on 06.15.2011 7:28 AM PDT]