Robert Harris' novel, "Dictator" depicts Julius Caesar as a cold psychopath:
"A vast but peaceful German migration of 430,000 members of the Usipetes and Tencteri tribes crossed the Rhine and was lulled by Caesar into a false sense of security when he pretended to agree a truce; then he annihilated them."
The Ancient Origins website gives a different figure (150,000) but notes his ruthlessness, killing the women and children first:
“I sent the cavalry behind to them.
“The Germans heard screams behind them, and when they saw that their wives and children were slain, they threw down their weapons and ran headlong away from the camp.
“When they had come to the point where the Meuse and Rhine rivers flow together, they saw no good in further flights.
“A large number of them were slain, and the rest fell into the river, where they died overwhelmed by anxiety, fatigue and strength of the current.” — Caesar, De Bello Gallico Book 4, 14-15
Naturally, Caesar puts a different slant on the migrating tribes, telling how they killed members of another tribe in their way on the far side of the Rhine, and claiming that the requested truce was only a ruse to make time for the Germans' cavalry to return to their horde.
Caesar also alleges that they attacked an advance party of the Romans, so his genocidal massacre was merely a pre-emptive (or preventive) strike to save losses to his legions. Coincidentally, I read today a review of a book about American neoconservatives who took this line with Iraq's Saddam Hussein:
"Saddam was not seen as a rational actor that could be deterred. Therefore a pre-emptive war was necessary to remove him from power. Fukuyama argues that America actually carried out a preventive war. Pre-emption is to stop an imminent attack, which was not the case in Iraq. Preventive is to stop a long term threat, which was what the administration thought Iraq was."
In Caesar's case, the use of the sword was not to spread democracy - he was soon to subvert the half-thousand-year-old democratic Republic of Rome itself - but to get greater power and the glory of a "triumph", which was only awarded to those who extended Rome's territory.
Frankly, I think the Senate couldn't have stabbed him soon enough.