Let’s take a moment here to consider what is going on. The Doctor has wired the room where all the original generation of Daleks are contained with explosives. They are only squids, so, defenseless. By connecting those wires he makes the whole room explode, ensuring the Daleks will never exist.
Of course, he won’t do it. Look at his face. The simple though of commiting this genocide revulses him. He calls himself no better than the Daleks themselves, if he goes through with it. Of course, it’s the mercy of the Doctor. He is aware of the consequences of his actions, but even then, he knows that if he stops being a healing force, if he stops being the Doctor, he could do more damage than any enemy he faced.
Flash forward 30 years later into the show. The Doctor, with his own hands, ended the Time War, by destroying both the Daleks and the madness driven Time Lords, committing a far wider genocide, and failing to eradicate the threat of the Daleks. He’s now the last of his kind, lonely, heartbroken, guilty.
Should he have crossed the wire ? Or is the innocence of his relative youth precious enough to warrant the steep price he would come to pay ?