Nadir

From wiki:

"In 1770, when the Maratha armies of Madhavrao Peshwa invaded Mysore (three wars were fought between 1764 and 1772 by Madhavrao against Haider, in which Haider lost), Haider expected British support as per the 1769 treaty but they betrayed him by staying out of the conflict. In 1780, he befriended the French and made peace with the Marathas and the Nizam. However, Haider Ali was betrayed by the Marathas and the Nizam, who made treaties with the British as well"

...as title say, why British betrayed Haider?

kauchenvinci-0

I suppose it's pretty much clear .
The british wanted the local rulers to fight among themselves regardless the cause .
They just took the path of watch , wait and interfere
watching them fight , allow them to exhaust and then interfere and dictate terms .
That was the way how they subjugated the nizam who always felt insecure due to marathas and mysore .
And once you get hyderabad , remaining was a piece of cake .