I'm currently in the United States but I will be leaving in a few days. I just realised that some bug spray (insect repellent) spilt in my suitcase. Is the spilt spray a big issue? Will I need to buy a suitcase? Not a lot spilt and I already disposed of the original container.
There shouldn't be a problem from a customs and immigration or airline handling perspective. Just wipe it clean and call it a day. Worse things have spilled from customer luggage (dirty diapers, colostomy bags, etc. etc.).
However..... if it was DEET based, then you should know that DEET will weaken nylon and other plastics. You should keep an eye out for any fabric that was saturated, especially if you had soft-sided luggage.
Probably not, check the luggage contents for stains. If there are stains, wipe them down with warm soapy water as directed from http://www.howtocleanstuff.net/how-to-remove-off-mosquito-repellent-stain-from-polyester
Otherwise wipe down with warm water to remove the residue and it should be good to go.
Given that for some countries at least it's actually recommended (maybe not officially, but I've seen it in papers from travel companies and books) you spray your luggage to prevent mozzies from hitching a ride with you and spreading things like malaria, I'd say it should be no problem.
Of course DEET (the active ingredient in a lot of bug sprays) can damage plastics when wet (softening the plastic, many people end up with fingerprints on their cameras when spraying their hands with the stuff, then picking up the camera to take pictures), so best make sure you don't touch it until it's dry.
When in doubt, clean the suitcase and that should get rid of the spill.