He's not really family material Where as the Venom Spin-off can be dark without being weighed down by Children going to see it, Problem with Carnage is he is to one dimensional kill, kill, kill and very unintelligent, he is just a way of getting more money from people because everyone liked venom although there has not been a Spider-Man villain like him he could prove interesting as for me I won't see it if Carnage is in it not even if he teams up with Mr. M mostly because well I hate Carnage
You have to remember that if it is used it was not part of venom if most of the mass is destroyed and if Eddie did survive there is no way he would know of that sample to look for it break in, steal it then that tiny sample could not completely cover him, now I know that symbiotes over a period of time can regenerate but he won't be in the movie following peter thinking only 20 hours before I can fight him again, also If the Director Hated Venom it's safe and sadly to say Symbiotes have had there time in the sun however could another symbiote been stored in that meteorite.
Another thing is Samples can be destroyed