I think symbiotes are not evil, they just reacting to their hosts' emotions. Venom was just reacting to Brock's thoughts and emotions toward Spider-Man when it bonded with him. Carnage reacted to Kasady's desire of kill people, that's exactly what the symbiote done along him. Toxin reacted to Mulligan's desire of protect innocent people, thinking he is cop, that's how became into one few heroic symbiotes on the marvel universe. 
 
The symbiotes are psychic, and can communicate with their hosts if they or their host want. When they bond with a host they gain all its memories, thoughts, and emotions, example: Venom gain the hatred that Eddie Brock felt for Spider-Man when it bonded with him, Carnage became insane when it bonded with Kasady 'cause of his desire to kill, Toxin became a hero when it bonded with Mulligan because he's a police officer. 
 
Many years ago Galactus ate one of the planets where symbiotes were living when they they came to that dimension, fortunately the symbiotes survived but they were left with a trauma that gave origin to the weaknesses against high vibrations and strong heat. The symbiotes are really scared by that, not harmed. 
 
In one point carnage bond with the silver surfer and made him attack people. That's because when a symbiote separates from its host it cleans up the negative emotions but stayed with it's host's memories and thoughts. But  'cause the trauma originate on its ancestors, the carnage symbiote would take more time to completely clean up the negative emotions within it, and when it bonded with silver surfer Kasady's evil desires and emotions were automatically transmitted along with his memories through carnage and take control of both of them. On other hand Kasady's memories could be transmitted to surfer's mind and automatically experienced all his bad life making him react to it and attack people. when silver surfer separate from the symbiote, he forgot all the bad experience he lived when he was bonded with it. That demonstrates if a symbiote bonds with someone that can't control its emotions, and if the symbiote cannot clean-up the negative emotions of a previous host (altogether with the memories) that emotions will take him/her over and will believe is that host once that has experience its memories, and will began to do that the previous host used to do. That same happened to Ben Reilly when he bonded with carnage and Spider-Carnage. 
 
A host can control and communicate with its symbiote when it's calm and can control its mind and emotions. Brock could almost control the venom symbiote because he was nearly calm and could control his emotions. A symbiote can amplify that emotions and desires 10 times. That can turn the host too insane that makes for impossible the symbiote take control of the mind of the host and his emotions and instead that evil emotions take control of both of them.  
  
The symbiotes have no intention to harm people, they just being manipulated by their hosts or any other being who wants to use their power for evil purposes. 
 
The trauma caused by Galactus in the symbiotes make they lose some control of his mind powers. That caused that the symbiotes couldn't purify their hosts' desires and emotions, causing all the madness in that specific group of symbiotes.  
 
Marvel Comics altered some characteristics of the symbiotes to make look bad, and make think people that  that are parasitic and evil and I don't agreed with that.