It has no, or extremely muted, emotion and no ego. And the Crucible is the cumulative effort of multiple cycles; once it's been completed by this one, it'll be completed by the next one, and the next again, and so forth, and eventually someone will win. The Catalyst can see the end in sight, so it'll just end things now to preserve as much as it can.
The "no emotion" part is debatable, I certainly don't see a clear indication for it.
The Crucible depended on the Catalyst to operate it (or tell Shepard what to do), and on the citadel to connect with it.
More so, it seems to me that the Catalyst actually allowed the the giant microphone to connect to the citadel, it would have been very easy to send a few
Reaper dreadnoughts to carve it to pieces-war assets or no, remember what it took to destroy Sovereign during ME1, and how easily it smashed apart
everything that stood in its way.
And if that's not enough, the Catalyst can just modify the Citadel to make it incompatible to the Crucible design.
It can actually try to destroy all the data about it, and it can actively prevent anyone from trying to rediscover it even if it was a threat. (which it isn't)
(Side note: It is rather obvious to me, that the idea that untold number of races designed a Frankenstein-machine over millions of years that was actually capable of changing the fabric of reality without actually understanding the machine itself is absurd to say the least.)
Anyway, by destroying itself, or letting someone inferior control the system, it's going against its original mandate and preventing itself from ever finding a solution to the "problem".
Shepard (supposedly) presented an anomaly (rather debatable as well), you don't solve the anomaly by breaking all your gear and shooting yourself in the head, rather you try to find a better solution. You simply wipe the board and start again.