The relay coil will also have a kick. When a coil is switched off it will produce quite a high voltage if there is no place to dump the stored magnetic energy. Even a 30 amp cube relay will generate enough voltage to give you a little shock. Without the TVS the voltage generated will fry parts designed for a max of 35 or 50 volts. A nice bi-directional TVS should have come with your Zilla -- it is absolutely important you use it . They are only about $1 part at Mouser of Digikey (1.5KE24) and the Hairball will be damaged if you don't use it. A regular diode should never be used in place of a TVS as they greatly slow down the contactor opening speed.