What is Slim suggesting in Chapter 6 when he says, " Never you mind...A guy got to sometimes"?

Slim says this after George shoots and kills Lennie at the end of the book. George is lying in the sand by the water when Slim comes and shows his support by sitting close beside him. When Slim says to him a person has "got to sometimes," he means that we all have times in life when we have to carry out a task we don't want to do, something that breaks our hearts but is necessary. He is trying to lessen George's guilt at killing his good friend, knowing that George did it to spare Lennie further agony.
It is important to remember that Slim is one of the most respected ranch hands. He is a wise and empathetic character who can feel what others are feeling and sympathize with them. He is describing as having hands which are:

large and lean . . . as delicate in their action as those of a temple dancer.

This suggests that while strong and masculine, Slim is also sensitive. The narrator also indicate that Slim is superior to others on the ranch when he describes him having

a majesty achieved only by royalty and master craftsmen.

The book ends with a faint glimmer of hope because Slim invites George out for a drink and George accepts. Slim is the only character who understood that Lennie had a gentle nature and did not intend to kill Curley's wife. George will now probably spend his money on drink and women rather than save it for a farm, but Slim will help him over the first, raw period of his grief.
Having Slim, a character of authority and sensitivity, show he understands why George killed Lennie helps readers too to understand that the shooting of his friend was an unfortunate burden George had to bear. George is as much a victim of a cruel world as Lennie—or Curley's wife for that matter.
All in all, Steinbeck depicts a cold world in which people who don't "fit" can expect very little compassion. Slim realizes that George was faced with hard choices and had to do the best he could to help Lennie.