It is in a cat's nature to reproduce -- by depriving him (or her) from that action, it can cause physical problems down the line with tumors, cancers, etc. (Cats would be susceptible to prostate cancer, breast cancer, ovarian cancer -- surprised me when I looked it up on the web.) Behavior problems are only the tip of the iceburg. Yeah, you NEED to get pets neutered or spayed. The older they get, the worse they get because of being deprived of their natural functions. I KNOW you had those males neutered -- who would want them "spraying" inside a house? Just take care of it early before they establish bad habits. And if it's a dominance issue, watch out to make sure the "loser" isn't banned from the litter box by the alpha. After several carpets and carpet cleaners, I learned the hard way that there is a definite need for more than one litter box and having NO large inside houseplants. Sometimes, too, they will attack each other if one is ill -- I had two male tabbies that kept trying to stalk and bring down an elderly shih tzu during the end stages of his life (cancer and urinary problems). It's all about survival of the fittest. They smell the weak, and nature kicks in. And when one of the tabbies started into old age renal failure, it's lifelong buddy turned on him. So you might need to get the victimized cat a good checkup if the behavior continues after neutering.