It's very easy to give bacteria certain antibiotic resistances, the mechanism is pretty much identical to the one used to make them green fluorescent in these kits. You transform the bacteria with a plasmid, they take this up and produce proteins from the genes on it. Those plasmids in the kit almost certainly have some antibiotic resistance on them anyway, that's the way you filter out the succesfully transformed bacteria.

The good news is that the resistances you can give this way are typically present in nature already. Bacteria are already exchanging these kinds of plasmids. And the bacteria you get in these kits or generally use in the lab are also harmless to humans.

I don't think you could effectively design a bioweapon with the stuff you have in such a kit. That would require a lot more knowledge, effort and material. I think at best you'd be able to create a resistant bacterium similar to those that already occur sometimes in the wild.