First I will say that the thought of my son smashing a frog makes me very sad. He has never done anything like that. He does not kill spiders or worms or bugs- although he swats flies for my mom sometimes. Once we were outside & he stepped on an ant & I told him I thought it was disrespectful to step on a bug when WE were the ones in HIS home... He is now a major bug rescuer & will come & tell me when he finds a spider in the house, before my mom gets to it, because she usually kills them.
Having said that, I do not in ANY way think that your son is messed up, sick, or twisted for what he did. I think he is young & he does not really comprehend that dead is dead & that animals- even frogs!- feel pain. I think as long as you continue to tell him that it was wrong, that will go a long way to ensuring that he does not do it again.
When I was little I pulled the legs off a daddy long legs because they continued to move afterwards- & I can remember squishing gypsy moth worms because their guts were bright green. I am not proud of that & I have not told Joe about it but I am telling you, just to reassure you that not every child who hurts small creatures will grow up into a cold blooded serial killer. Jeffrey Dahmer, yes, but how many more times have we heard the neighbors of a convicted murderer say, "He always seemed like such a NICE boy?..."
I can remember the boys in my 9th grade gym class hitting a turtle with a baseball bat, & I am getting sick to my stomach just typing that. I can't understand THAT type of cruelty AT ALL, but I don't think your son belongs in that category!
I was thinking, IF he does this again, what do you think he would do if you burst into tears? Because I know when Joe stepped on the ant, he could tell by my voice that I was appalled. It had a big impact on him I think.
Good luck to you, & please don't worry about your son growing up to store body parts in his basement. I can only imagine the struggles you have had, raising a son with bi polar disorder- my mom's 70 year old friend has this & it is a very hard illness to cope with.