Why Not Put Catiline to Death?

Cicero

Catiline, the man who plotted to kill not only Cicero, but also a large portion of the senators. Even today, people commit crimes and are punished accordingly by authorities. However, during Cicero's current time, he states "At vero nos vicesimum iam diem patimur hebescere aciem horum auctoritatis", a metaphor for how  

their authority is getting weaker and that they're losing power. In order to fix this lack of power, Cicero suggests that Catiline should be punished (and so be put to death) for his crimes immediately, as any other man who has committed a crime. 

Cicero blames himself for not having done anything about Catiline sooner. He says it allowed Catiline's camp in Etruria flourish with the amount of soldiers growing day by day, claiming, "Castra sunt in Italia contra populum Romanum...die singulos hostium numerus". To follow that up, Cicero essentially calls out Catiline and points out that he is the leader of that camp that plans to take over Rome, but he's right there at the meeting.

Further down the speech, Cicero begins to claim how he is conflicted on what he should do about Catiline, since both killing him and not killing him has consequences. "Quam diu atque erit qui te defendere audeat... speculabuntur atque custodient." Eventually, Cicero decides that he'll allow Catiline to live for as long as he has supporters, and he'll been watched carefully from now on.