Converting hexadecimal digit to two's complement
To get the 2's complement you must define the number of bits.
The most significant bit is the sign bit.
So if the number of bits is 8 then you should get:

14 is 00001110 (MSb=0 : positive)

-14 is 11110010 (MSb=1 : negative)


if the number of bits is 4:

you cannot code +14
+2 is 0010
-2 is 1110

2's complement of A is 2^n-A which we can get also if we apply: 1's complement + 1 (since the 1's complement is 2^n-1-A)
That is why 14 as raw binary is -2 as a 2's complement on 4 bits (it is 16-2 since 2^4=16) and there is a reason for that. In fact, the goal is to get a negative number coding where we can still have right results when applying basic operators like adding for instance on negative numbers, positive numbers and a mix of them.
Note: MSb is the most significant bit. Do not confuse with MSB (most significant byte).