In this guide, you will get to know how to kill Lambert. It is a little surprising, but you can kill Lambert in Witcher 3.
The first key to Lambert’s death is not to have Keira Metz, who usually saves him from the wild hunt. To do that, you will need to skip the part where you chase her back to the Fyke isle. And the other way is to call her a two-headed snake, which will offend her. 
You can also kill her; it will ensure she will not save Lambert.
The other way is during the actual battle, when the time comes, you can sit back and watch the wild hunt killing him. 
If you want to see if Letho can save him, it is not happening. Lambert will still get killed, even if you intervene or not.
After that, you will be alongside Vesemir, and Geralt has a short line when interacting with Py
