People are capable of more than 1 emotion at a time and that doesn't make any other emotions invalid and it certainly doesn't make any of the other possible ones people may feel or express "immoral".

Yes, someone is dead. They were flawed, but so is everyone and their passing is a tragedy to those close to them. Sadness, mourning, and empathy for those who will be most affected by his passing is a valid emotion.

Ryan actively chose a profession dedicated to inflicting violence upon those around them including the deaths of many others. Relief that he is no longer able to cause harm is a valid emotion.

Ryan was unable to stop causing harm to others on his own volition. He likely did it with the best of intentions, but through a steady diet of misinformation and lies he was conned into acting as a violent enforcer of capital. Frustration that this is what it took to prevent him from inflicting further harm is a valid emotion.

The empathy you are demanding with "it's bad when people get killed" is the same moralistic argument that "it's good when killing is avoided". Celebration that Ryan will no longer be causing the deaths of others is a valid (and morally equivalent) emotion.

Etc.

In short: It's a good platitude, but it's a poor moralistic argument, and is a narrow-minded viewpoint. Lemmy isn't the problem, your lack of empathy for those outside of Ryan's direct social circle is.