apt - How to rollback to an older kernel on 16.04/purge kernel automatically installs new one?

unattended-upgrade recently automatically upgrade my kernel to 4.4.0-143 but it does not work with Nvidia graphic cards so I decided to roll back to previous version.

I am able to boot to an older kernel(4.4.0-141), and confirmed I'm booting my computer with that:

4.4.0-141-generic

but when I tried to apt purge linux-image-4.4.0-143-generic, apt prompts me to confirm:

The following NEW packages will be installed:
  linux-image-unsigned-4.4.0-143-generic

So why is uninstalling a signed kernel requires to install an unsigned version?
And how can I safely remove the newer(143) kernel? Thanks in advance!

full output of apt purge:

Reading package lists... Done
Building dependency tree       
Reading state information... Done

The following packages were automatically installed and are no longer required:
  amd64-microcode intel-microcode iucode-tool linux-headers-4.4.0-137 linux-headers-4.4.0-137-generic linux-headers-4.4.0-138 linux-headers-4.4.0-138-generic linux-headers-4.4.0-139 linux-headers-4.4.0-139-generic linux-headers-4.4.0-141 linux-headers-4.4.0-141-generic
  linux-image-4.4.0-137-generic linux-image-4.4.0-138-generic linux-image-4.4.0-139-generic linux-image-4.4.0-141-generic linux-image-extra-4.4.0-137-generic linux-image-extra-4.4.0-138-generic linux-image-extra-4.4.0-139-generic linux-image-extra-4.4.0-141-generic
  linux-signed-image-4.4.0-137-generic linux-signed-image-4.4.0-138-generic linux-signed-image-4.4.0-139-generic linux-signed-image-4.4.0-141-generic thermald
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
  linux-image-unsigned-4.4.0-143-generic
Suggested packages:
  fdutils linux-doc-4.4.0 | linux-source-4.4.0 linux-tools
The following packages will be REMOVED:

  linux-generic* linux-image-4.4.0-143-generic* linux-image-generic* linux-signed-generic*
The following NEW packages will be installed:
  linux-image-unsigned-4.4.0-143-generic
0 upgraded, 1 newly installed, 4 to remove and 140 not upgraded.
Need to get 7,049 kB of archives.
After this operation, 386 kB of additional disk space will be used.
Do you want to continue? [Y/n] n