Repeating it tonight in front of Death Eaters who either witnessed or heard about the first occasion was close to suicidal!"
Harry Potter and the Deathly Hallows
I am wondering why it's "close to suicidal" instead of "close to suicide"?
Suicide is an activity someone does. You can say "close to suicide" to mean someome is soon going to kill himself/herself.
Suicidal is something that someone is, and it usually means someone no longer cares if they live or die, therefore he/she acts reckless. It can also mean "considering suicide" but doesn't necessarily mean a person is about to kill themselves in the next moment.
A very reckless activity, one that has no regard for any consequences, can figuratively be described as suicidal.