Because Jarasandha wanted to sacrifice several kings for selfish purpose. This is utter cruelty.
In past the kings used to invade other kingdoms. But usually that was meant for extending their borders. Later the invaded king, if alive - would surrender to the winning king. This is how the normalcy of the people of the kingdom being ruled won't be affected. Because the local kings would know better, how to run the political affairs.
In case of Jarasandha, he had imprisoned several kings and had a wicked plan of slaughtering them - like animals. Imagine the chaos generated in those cities.
If he had not been stopped, his such 'yagna' would have extended towards Kuru & Yadu kingdoms as well.
From Jarasandha badha parva, Jarasandha himself also used to think of him as innocent, when he met Krishna along with Bhima & Arjuna, disguised as Brahmana-s. When they disclosed their mission to kill him ...
Jarasandha said,--'I do not recollect if I ever acted injuriously towards ye! Even upon a careful mental scrutiny I fail to see the injury I did unto ye. When I have never done ye an injury, why, ye Brahmanas do ye regard me, who am innocent, as your foe?
Krishna said,--'O thou of mighty arms, there is a certain person of the head of a (royal) line who upholdeth the dignity of his race At his command have we come against thee. Thou hast brought, O king, many of the Kshatriyas of the world as captives (to thy city.) Having perpetrated that wicked wrong how dost thou regard thyself as innocent? O best of monarchs, how can a king act wrongfully towards other virtuous kings? But thou, O king, treating other kings with cruelty, seekest to offer them as sacrifice unto the god Rudra! O son of Vrihadratha, this sin committed by thee may touch even us, for as we are virtuous in our practices, we are capable of protecting virtue. The slaughter of human being as sacrifice unto the gods is never seen. Why dost thou, therefore, seek to perform a sacrifice unto god Sankara by slaughtering human beings? Thou art addressing persons belonging to thy own order as animals (fit for sacrifice)! Fool as thou art, who else, O Jarasandha, is capable of behaving in this way? One always obtaineth the fruits of whatever acts one performeth under whatever circumstances.
Here is the video from BR Chopra's Mahabharata discussing the same above.