Can LOOP-Programm stop when its value goes below 0?
Variables in LOOP programs are non-negative, so the problem never arises; the instruction $x_1 = x_1 - c$ assigns $x_1$ the value of $\max(x_1-c,0)$.

Note that the instruction $x_1 = x_1 - x_2$ isn't allowed: the second operand has to be a constant. However, since $x_1 = x_1 - x_2$ can be implemented in LOOP, the programming language "LOOP with instructions of the form $x_i = x_i - x_j$" has the same power as LOOP.