Let's go over the heist:
Scott Lang's pal sneaks into the crowded Pym Technologies and lowers the water pressure so our tiny hero can swarm through the bathroom sink like some icky phantom. Lang then uses his ant army to fry the servers containing all of the bad guy's backup data before attempting to fly through a security grid and steal the Yellowjacket suit -- effectively eliminating the company's ability to re-create the invention. Sounds good, right? But just in case there are any unforeseen problems, Ant-Man also does this:
Earth's tiniest hero, but largest dickhead.