Is he or was he making a bubble nest? I have two Bettas in tanks next to each other and usually it's not a problem. They would occasionally do a little flaring when they noticed each other and then go on interested in something else. But today Red was going nuts, it wasn't the same "look at me I'm prettier than you" flaring but more like "I'm going to rip you apart if I can just get over there". I ended up having to put a paper up where Red couldn't see Blue anymore.
Then I found the nest poor lonely Red is working on. Red and Blue both have made at nests before but Red is taking this one seriously. It's big and he has it in between some plants. It stands to reason that he would get more territorial if he has breeding on his mind, males of all species do right?