Polarities when expressing currents in terms of node voltages
You can pick either polarity.
You can pick different polarities at different nodes.
You, however, must sum to ZERO the currents either

flowing INTO the node

or

flowing OUT_OF the node.

Thus I'd write that ---- summing currents INTO the node --- as
0 = I2 + (V1 - Vth)/R2 - (K * I) + I
but I'd redefine I as [ - Vth/1]