When it is true that association with bad people and bad things can drag people towards bad path, there is also a belief that good people always stay good in spite of their association with bad people. If good people are associated with bad people, the good qualities are usually transferred from the good person to the bad person. Even in some rare cases if good people are influenced by bad people, one day the true nature of good people will surface.
विकृतिं नैव गच्छन्ति सङ्गदोषेण साधवः |
आवेष्टितं महासर्पैश्चन्दनं न विषायते ||
vikrutim naiva gachhanti sangadoSheNa saadhavah
aaveShTitam mahaasarpaishchandanam na viShaayate
The virtuous do not get affected by the company of the wicked. Just like the Sandalwood tree does not turn poisonous in spite of being surrounded by the venomous snakes.
Sometimes association with wicked people might be necessary to learn how we should not be. In spite of their association with bad people, good people will still try to spread positive energy in the surrounding. Vibhishana from Ramayana is the best example for this. Though he was surrounded by lots of Rakshasas including his brother, his true nature was evident until the end. That is the reason why he was able to recognize Rama as the supreme soul and make associations with him.