Shimada seems to have been a gang member only by default. His life seemed to be one of aimlessness, so the only avenue of interest was the goofy and irresponsible things that Shoya did. He just 'went along for the ride'. He did, however, see the wrongness of the bullying that Shoya did to Shoko, but it was much later that he would take a stand. He turned on Shoya, and only then condemned the bullying, making his judgement somewhat hypocritical.
On the night that Shoya fell from a third story balcony, and fell into a pond of cold water, bleeding profusely, Shimada was one of the two boys who fished Shoya out of the water and got him medical attention. This shows he did have a grain of compassion and common sense with regard to Shoya and was willing to put personal issues aside for the common good. Later, however, when he was in high school, Shimada worked part-time at an amusement park. There, he sees Shoya for the first time in a while but refuses to acknowledge him. He only serves Shoya from the booth he was working in with no recognition or acknowledgment, treating Shoya as an unknown typical park customer.