I know Zazu doesn't appear to be evil in Lion King, but what I found on this reddit here is intriguing:
https://www.reddit.com/r/FanTheories/co ... r_scar_to/
Let's put it this way, Zazu may have wanted Simba out of the way, thinking he'd be a terrible king. Now, it's possible that Zazu never intended that Mufasa died, but wanted Simba gone.
If Zazu had met up with Scar (possible since he is off screen for a bit and then turns up in the Elephant Graveyard to scold Simba and Nala (we also know that Scar was in the Elephant Graveyard too but we don't know for how long, he could have crossed paths with Scar earlier.) It would also explain why the hyenas never tried to kill Zazu along with the other two: he was in on it. Let's face it, if they'd succeeded in killing Simba and Nala, if Zazu WASN'T in on it, he'd start naming names and all of Pride Rock, not to mention Mufasa, would descend upon Shenzi, Banzai, and Ed and rip them to shreds for killing the two cubs.
Also, interestingly, Mufasa was NOT off on some mission to fight the hyenas in the Gorge scene, but was NOT with Simba, which in and of itself was odd considering that one would think Mufasa would have kept a very close eye on Simba. Now, it's possible that Mufasa may have trusted him with his brother, but even Mufasa knew that Scar didn't really care for Simba. (Too bad he never told Simba that!!!)
Mufasa is seen with Zazu during the gorge incident. With if Zazu had distracted Mufasa , knowing that the stampede would happen?
Also, when Zazu finds Simba, Simba cries "Zazu help me!" and Zazu says "Your father is on the way! Hold on!"
When Mufasa goes into the mess himself, Zazu cries "Oh Scar this is awful! What will we do? What will we do? I know, I'll go for help!"
However, that seems odd that he didn't at least try and grab Simba (like Simba asked) to get him to safety. If Zazu was in fact in league with Scar, this would make sense. Zazu probably wouldn't want Mufasa dead and would start to panic, worried that Mufasa might die in the stampede and wondering how he was going to explain himself. Perhaps "Going for help" was his way to establish an alibi. At any rate, it's possible his alarm (about Mufasa at any rate) was legit. When he'd found out that both Mufasa and Simba had died in the stampede, he felt guilty, having plotted with Scar to kill Simba.
Of course, Zazu, having been knocked out, wouldn't have known that Scar directly murdered Mufasa, and would have thought that his scheme with Scar led to Mufasa's death. As such, perhaps Scar played the guilt game on him too and this kept him from ever ratting out Scar.