They have no spores or anything (at least wiki doesn't mention anything like that), the only way to infect people is direct blood transfer (different sizes for different success rates and speeds).
So the slaughter
did have a point: they were collecting bodymass for their bioengineering of the XVI-H strand, as well as gathering the very early volunteers (if there were any). That's the very point of the early attacks, gather enough humanity to research and develop a suitable strand of XVI. They continue this for some time after. I'm guessing we gave them too much resistance and forced them to escape prematurely.
After they realise that we could actually
hurt them, they decide to do things more covertly in order to minimise resistance (this is why I'm guessing they never wrote books about strategies and tactics like we have [Sun Tzu notably], they didn't know much about our strength and proceeded to underestimate us). Basically, the initial attacks raised the interest and anger of humanity and doomed the aliens (if PHALANX succeeds, of course).
After that, both parties (MUCH more emphasis on the alien side) realise how big of a fight this will turn into. I think the local XVI-aliens worked pretty cut-off from the main planets (for various reasons) and as such have to specifically start to order more heavier weaponry to be shipped here (heavier plasma weaponry and particles).
Edit: Basically, what I'm saying is that if the XVI had been more wiser about strategies, they would have done things much more similarly to the
Yeerks of the Animorphs series: start slowly by gathering volunteers (and covertly doing the forced infections), then bit by bit get more brave do more riskier things, then
and only then start a full-on invasion, hopefully with one or more supranation under your control. Now, the XVI ran straight into the eyes and sights of humanity and pays the dear price (again, supposing that PHALANX succeeds).