In your project if you point out positive qualities of Prithvi, then your teachers who will evaluate your projects will point out his negative qualities. This is because the teachers would have read the books written by the historians who criticize Prithvi.
Therefore, find out exact reasons what arguments these books give against Prithvi. And then think what arguments you have against their arguments.
When we talk of good and bad acts, then any act can be good or bad depending on two things - 1. intention 2. result.
The same thing can be good according to intention and bad according to result, and vice-versa.
Prithvi forgave and released Ghori. Prithvi's intention was good - in fact very noble. But the result was bad. This is because Ghori cheated Prithvi. However, one must keep in mind that if we do something, then we can only guess what the result will be - we cannot know it exactly because nobody knows future. Knowing what will happen is even more difficult if what will happen depends on what somebody else(and not what we alone) do. In this case, the result depended on what Ghori did. If Ghori had really made friendship with Prithvi, then historians would be praising Prithvi.