CherryPrinces and Megha....your thinking are to some extent right that Prithvi's approach can't be fully justified!
But, I think, the truth comes from the mythology period that "to kill bad..., you have to become bad...some times, otherwise, you may not be able to kill the bad..."
Example....Pl. recall how Duryodhan was killed by Bhim in the 'gada yudh' in Mahabharata - After Shri krishna's ishara, Bhim hits at the waist area (that is against the rule) and thus Duryodhan died because he got some power from his mother Gandhari that he can't be injured any where else than his waist area....
But, kehte hen na, paap ka ghada ek din bhar jata hei....so duryodhan had to die then, and the devil could only be killed by trick. Will you say here that, Duryodhan did tricks and got some power, but still then, Bhim shouldn't have done any tricks and may be, would have died in the hands of Duryodhan? That wouldn't have been justified either, right?
....In this situation, Bhim's trick can be justified better (if not fully) than Duryodhan's trick.
I feel a same kind of situation between Prithvi and Bhimdev here....Prithvi's trick is to kill a devil in the form of Bhimdev, whereas Bhimdev's effort is to ruin/kill Prithviraj who's no devil to any one.
Now, about expanding wealth being the dharma of a king, I think, it's said for those Kings who went on straight wars for such purposes...and won kingdoms. But if you use tricks for that....that's deceiving...and can't considered as a noble way!
At the end, I want to request Cherry and Megha to do some research and find out that if the Bhimdev killed by Prithvi was the King Bhimdev remembered for his noble did in Gujurat?
From an article (that I forwarded to Cherry) what I understood was ... Bhimdev killed by Prithvi was also named as bhola Bhim and he is not recognized in History for any significant dids (good or bad)!
The Bhimdev who is recognized in History was ruling after about 100 years from Prithvi's death! (...and both Bhimdevs were called as Bhimdev Solonky)
Since Prithvi is remembered for his nobleness and kindness, if he killed king - Bhola Bhim, then its natural to conclude that king bhola Bhim was bad to that extent.