Actually soldiers top kills a lot, in fact they do it more than all other classes in typical premades (if there's 2 soldiers or more per team, when there's only one he might be flagging and not killing much) but even in pugs where casters can farm baddies, soldiers can still top kills if they want to. And even when they get slightly less kills... they don't die nearly as much and their k/d ratio is so much better it's not even comparable.
Going for kills with a soldier is the easiest and dullest thing ever. It's also very relaxing though, which is why I do it from time to time. But playing the flag and objectives is usually a lot more fun (I know, doesn't always work with pugs) But even then, seen plenty of time tanks (me included) flagging AND topping kills in pugs (and I do have a bunch of screenshots)