Externally, Lennie's strengths are that he's a great worker and his intentions are completely harmless. Unfortunately, he doesn't know his own strength and his unwittingly kills mice and rabbits in attempts to pet them. His strength is therefore a strength and a weakness.
Internally, Lennie is equally as innocent as he is externally. He only wants to appease George and someday get their own farm, with rabbits of course. Lennie's internal and external conflict is caused by his mental handicap. The problem is that his lifestyle as a ranch hand was just not hospitable for someone who thinks like he does. And when he gets nervous, he literally doesn't know how to control himself mentally or physically.
Externally, Lennie's conflict is that he's a great worker but that which makes him a great worker (his strength) is also what makes him unintentionally violent.
Internally, Lennie's conflict is that he has nothing but benevolent thoughts, but he knows in situations where he feels threatened, he can only react instinctively, like an animal, and it's during those times when he can become extremely dangerous.