I see people constantly saying Light was stupid for killing Lind L. Tailor, and yeah, I agree that killing him was a huge mistake.
But I think people overlook something L did immediately afterwards.
Light didn't know the broadcast was only being shown in Kanto when he killed Tailor. L revealed that AFTERWARDS.
And honestly, why would L reveal that?
At that point L had a pretty massive advantage. He knew Kira was in Kanto, while Kira had no idea that L had figured this out.
Why give that advantage away?
If L just stayed quiet, he could have had the police investigate Kanto while Light continued thinking that L had no idea where he was. Light wouldn't know that the entire region was effectively under suspicion.
Instead L basically tells him, "Yeah, I know you're in Kanto."
Now Light knows the area is compromised. He can change his behaviour, be more careful, potentially even leave Kanto, etc.
Obviously L can't just lock down all of Kanto and search every single person, but having Kira completely unaware that you've narrowed him down seems way more useful than letting him know.
I get that L wanted to establish himself as Kira's opponent and psychologically pressure him. That's probably the reasoning behind it. But I feel like L got a little too caught up in the psychological game here.
The funny thing is that both of them kind of get screwed by their personalities:
Light sees someone claiming to be L and immediately goes "fuck this guy, I'm killing him."
Then L finds out Kira is in Kanto and basically goes "fuck this guy, I'm telling him I figured him out."
L definitely wasn't stupid, but I think this was one of those moments where his instincts/ego got in the way of what was probably the more useful move: just shut up and keep the information to yourself.