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. ...
No, Krishna was not incompetent against Jarasandha. In fact, Krishna and his army has defeated Jarasandha and his army eighteen times.
Krishna running away from Jarasandha implies that, Jarasandha was destined to be killed by Bhima in duet.
This story is mentioned in Harivamsa Purana (Khila of Mahabharata) as follows:
Realising that jarAsaMdha was skilled ...
"...from fear of Jarāsandha, at one time, we had to leave Mathurā", says Kṛṣṇa in Sabhā Parva of the Mahābhārata. I don't think it signifies anything else.
Kṛṣṇa explains to Yudhiṣṭhira as follows.
Then, O great king, remembering the conclusion to which we had come of old we became exceedingly cheerless and fled from Mathura. Dividing our large wealth ...
As told by another user in his answer, Incompetence is not at all the reason for Krishna.
There are two key reasons behind this
Krishna did not choose to kill Jarasandha to make his task easier and
Krishna obeyed the command of Brahma that Jarasandha has to be Killed by Bhima only
Now, I am providing the scriptural statements for both reasons from the ...