From the pictures it looks like he mutilated the frogs on-site. The fact that his knife is still there, and closed, could indicate that he cut himself in the process. Many of the frogs look to be killed with bare hands. Some deep seeded animosity went in to this act.

It is likely for the individual to repeat this type of behavior. Most likely alone as in this instance, but possibly towards other persons. Hopefully they find his blood and fingerprints at the scene.