Here we go. First, the class has no need to be abstract as it has no virtual members. In fact, since it has all static members, the class should also be static. Also followed C# naming conventions, in which local variables should be camelCased rather than the PascalCased you have. Used some LINQ to declare intent rather than implementation by using .Any() in a few places. Also re-declared List<Node> throughout as IList<Node> since programming to interfaces is better for decoupling. For that matter, wherever Node lives, may want to extract an INode interface from it and use it here. Finally, a tiny performance improvement: don't allocate the lists until after the base case has been evaluated (and returned from) so the GC doesn't have as much pressure on it.

namespace prjT02L08_Predator_Prey
{
    using System.Collections.Generic;
    using System.Linq;

    public static class MergeSort
    {
        public static IList<Node> Sort(IList<Node> input)
        {
            if (input.Count <= 1)
            {
                return input;
            }

            var midpoint = input.Count / 2;
            IList<Node> left = new List<Node>();
            IList<Node> right = new List<Node>();

            for (var i = 0; i < midpoint; i++)
            {
                left.Add(input[i]);
            }

            for (var i = midpoint; i < input.Count; i++)
            {
                right.Add(input[i]);
            }

            left = Sort(left); // Recursion! :o
            right = Sort(right);

            return Merge(left, right);
        }

        private static IList<Node> Merge(IList<Node> left, IList<Node> right)
        {
            var result = new List<Node>();

            while (left.Any() && right.Any())
            {
                if (left[0].F < right[0].F)
                {
                    result.Add(left[0]);
                    left.RemoveAt(0);
                }
                else
                {
                    result.Add(right[0]);
                    right.RemoveAt(0);
                }
            }

            while (left.Any())
            {
                result.Add(left[0]);
                left.RemoveAt(0);
            }

            while (right.Any())
            {
                result.Add(right[0]);
                right.RemoveAt(0);
            }

            return result;
        }
    }
}