Is KingRoot secure?

I have unlocked my HTC device and installed TWRP and SuperSU. Then I accidentally leaked root access to one of Tencent's apps. The log says that the app has leaked root permission to KingRoot SDK:

It uses root permission to execute something like 'kd -d krsdk.cert'.

Then I removed its root access in SuperSU, however the app is still able to use root access.

I then downloaded KingRoot, and it says that the device has already been rooted. I tried to unroot the device from KingRoot, however I could re-root it again with KingRoot. I tried restoring the device using a complete backup from the TWRP recovery (removing SuperSU and everything else just like factory default), but KingRoot still works.

Is that because my device actually has a bug, or because KingRoot has left a persistent backdoor in my device? If so, how to remove it?

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