Jealousy isn't pretty, but if you think that's the only thing that drives aiji do this, then I'm sure you're wrong.
No I’m a aware it’s more complex, Aiji and Seij Love i is painful and lonely to some degree, Aiji noticed subtle changes in Seiji and knows Towa’s the cause. Seeing Seiji and Towa becoming closer ignites jealousy, frustration and fear. He lashed out cruelly and Seiji steps in when he goes too far. I imagine that must have been painful seeing Seiji protecting someone other than him. Then hearing him mention other people’s thoughts when he tried to chain Towa outside only reminded him of all his insecurities. He must feel It would be so much easier if they were a pair.. Aiji is trying to be thoughtful in his own way. At least this is my interpretation
I think it's more than jealousy... But we wouldn't know until its revealed anyway
Seiji knew Aiji would be mad... Seems Aiji gave Towa a heat inducing drug... Jealousy isn’t pretty.