IMO the reason Issei is unable to control himself when he sees his comrades getting hurt or defeated has a quite a bit to do on how his as a devil started. He was place into combat situations almost immediately after being turned into a devil and the emotions and frustrations he felt during them.
Spoiler for :
In his first battle against Freed, he was easily beaten by him and had to watch in pain as Asia protected him from Freed. As well as watch as Freed hit Asia for protecting Ise and even threaten to rape her. In first fight with Raynare, he is once again protected by Asia by willing going with Raymare to spare Issei's life, even after trying to protect Asia from her. In second battle with Raynare and her group, he was too late to stop Raynare from taking Asia's SG and killing her in the process.
Volume 2, During the first meeting with Raiser, he had little skirmish and was easily beaten by Raiser's weakest Pawn and then then had to sit endure as Raiser calling him weak, laughing at him, and mocking him and his Boosted Gear. During the RG, he had to at times watch his comrades being taken out one after another. All the while knowing that if he lost, Rias(the girl he was to fall in love with) would be forced to marry someone she did not love or even like as well as stand because of her parents greed and wishes.
All these events happend almost one after another.
When you stop and compare them, Isse has been in way more real life and death battles than RG's. Issei is still young and is still not able to separate his emotions in real battles from those that are need for Rating Games.