How could Ronan the Accuser be killed by the Power Stone if he is immortal?

Ronan is not immortal. As seen on this Wikipedia page, his powers are listed as:

Abilities

• Expert in military tactics
• Skilled hand-to-hand combatant
• Superhuman strength, speed, endurance, and reflexes
• Utilizes Kree exoskeleton armor
• Wields "Universal Weapon"

It mentions nothing about immortality, nor does the page for the Kree. However, it is possible that he became "immortal" at some point, but see this Q&A to get some clarification on how he became powerful and why you might think he is "immortal".

When you say;

The Terrans defeated the Kree

I presume that you are talking about in the comics. The comic continuity is not relevant to the Marvel Cinematic Universe, which takes place in an alternate dimension (Earth-199999). As such, the Kree may be very different in the films from in the comics. We have only seen a couple of instances of Kree in the Marvel Cinematic Universe so far, so we don't have that much information to draw upon, but;

We know that in Guardians of the Galaxy, a truce has been drawn up between the Kree and the Xandarians, which implies that they were at war before. The fact that the Xandarians (who are not immortal, as we see many of them die in Guardians of the Galaxy) have been able to negotiate a truce with the Kree suggest that the Kree are not immortal either.

Also in Guardians of the Galaxy, we see that Ronan is able to hold the Power Gem (one of the six Infinity Stones) for a limited amount of time, before embedding it in his hammer. The fact he has to put it in his hammer implies to me that, like the slave girl that works for The Collector, it would have quickly destroyed him. He is also capable of great feats of strength, throwing Drax around like it's no big deal when they fight in Knowhere. So we can see from this that Ronan, if not neccessarily all Kree, are stronger than they might appear.

Additionally, in the Agents of SHIELD season 2 episode "Who You Really Are", we see Lady Sif (an Asgardian) fight a Kree warrior, and the two of them seem to be fairly well matched in terms of strength and durability. This would imply that it isn't just Ronan the Accuser who is strong, it is all Kree (or at least the ones trained in combat).

In short, the Kree are not immortal, but they are very strong, most likely due to their biology rather than any gained power.