Why was killing Julius Caesar a bad thing? Please stick to the topic that it was bad.

First, "killing Julius Caesar" is a topic. Arguing that this was a bad decision is a stance on a topic or opinion about a topic. The two things differ. 

In Julius Caesar by William Shakespeare the conspirators have a wide range of motives. Cassius seems motivated mainly by jealousy, Brutus by a desire to preserve the Republic, and others by various political and personal considerations. 

From a moral point of view, one can argue that...

First, "killing Julius Caesar" is a topic. Arguing that this was a bad decision is a stance on a topic or opinion about a topic. The two things differ. 

In Julius Caesar by William Shakespeare the conspirators have a wide range of motives. Cassius seems motivated mainly by jealousy, Brutus by a desire to preserve the Republic, and others by various political and personal considerations. 

From a moral point of view, one can argue that murder is always immoral. Even though Brutus had the interest of the Republic at heart, he killed a man who was a close friend and mentor to him, an act we might find morally bad. For Cassius and the others, we can argue that their motives as well as their acts were morally bad.

From a practical point of view, the death of Julius Caesar did not lead to the restoration of the Republic but to the beginning of Empire, with Octavius (Augustus) taking on the role of Emperor. One of the arguments that Brutus made was that because Caesar was a fundamentally good tyrant, he was more likely to cause the permanent end of the Republic than a bad tyrant against whom people would rebel. In actuality, though, he merely was replaced by another tyrant.