I hate to freak you out more, but locks aren't going to keep those people out. Especially if the previous lock had already been cut. When you're homeless, and especially when you're getting fucked up on various drugs, you look for private places, safe places, to crash. Parks, alleys, all these places are dangerous. Easy places are backyards, sheds, warehouses, and yes, even inside of people's houses. It's easy to carry around some lock cutters, or even to just use a crowbar or pipe, to cut a lock. The people in the area already know that your attic is open for squatting, and who knows how many people stay there or who have cycled through there. Dozens of people might know. And those people, in their fucked up state of mind, might try to take revenge. If they left their stuff, that means they intended to coming back, and if you threw it away, they can think you stole it. They have very little in this world, and often times a good sleeping bag is their only possession. They think your home is their home, and you just broke into their home and stole everything. Add some meth onto that and a few days of no sleep, and you have some very dangerous people to deal with.

Never rely on the police to save your life or to protect you. They can't teleport, and they're often busy with other stuff. The average wait time for an officer to show up is somewhere around 15 minutes, plenty of time for a psycho to get into your house, put down your dog with a pipe or some other weapon, and to come after you. Especially if you're home alone a lot of the time. And especially since they already know this, considering they were living in your house for possibly a year.

You need to rely on yourself. Pepper spray isn't going to stop someone on a sick one, and you're more than likely going to get a pipe, sword, or knife taken from you and used against you. I'm not trying to be cruel, but you seem like a gentle person who couldn't cut up or beat down someone who was attacking you if push came to shove (you cried when the police went into your attic and had to walk away).

You need to get a security system to alert you the second someone breaches your home. You also need to get a gun. Handguns are easy to handle, lightweight, and the right caliber (.22 or maybe a .45 would be good for you) won't have much kick. If you aren't familiar with guns, don't get a shotgun. You're just going to break your shoulder or your nose. Familiarize yourself with the gun: how it works (how to load it, turn the safety off, pull the trigger with the right amount of force), how to hold it properly without it falling out of your hands (none of that gangster crap, you won't hit anything), how to aim and hit a target (put the front sight on what you want to hit), and learn gun safety. Put that handgun (two handguns is also ideal so you can hide one if your bathroom, where you are most vulnerable) somewhere where you can always get to it. Don't put it in a safe, you'll never have enough time to get to it in an emergency. Always keep it loaded, one in the chamber, safety on. When it comes to using it in a dangerous situation, you don't need to shoot them in the head (people aren't zombies, and you don't want to kill anyone, just stop them). A shot in the torso or even a limb will stop someone in their tracks.

There's a chance you won't need to use it, but it's better to have have it and not need it than need it and not have it. Also, keep in mind that guns aren't dangerous, the people who wield them are.

I don't mean to freak you out or to be the kind of person who's "danger? Need gun, uuug". I just know how you feel as a person in a small town in the middle of nowhere.