Roman law certainly didn't forbid the death penalty. I'm guessing you meant 'in cases of adultery', though that's not fully correct either--Roman law from 18 BC on recognized the right of the aggrieved huband to kill his 'cuckolder'...just not his adulterous wife (of course under Mosaic law both could be executed, provided the limitations imposed on capital crimes trials by Jewish law were observed). Since only Roman authorities, not the Sanhedrin, held capital case jurisdiction in Roman Judaea, the likely "compromising situation" involved here is that Jesus would have violated those terms by rendering any "verdict" whatsoever on his own. Of course Jewish law wouldn't have permitted him to summarily declare a verdict on the spot, either, but presumably that wasn't per se what they were asking for--the conceit appears to be that they simply wanted him to 'rule' whether she could be executed if found guilty by trial or not, and technically either a direct "yes" or "no" could be considered a 'subversive' usurpation of capital-case jurisdiction on his part. Typically, he threw the responsibility for the decision back on them instead--in a way which insinuates without actually saying so that he recognizes an attempt to legally entrap him is afoot (potentially a capital crime in itself, under Jewish law).
Whether or not a "good Jew" at the time would have said (or thought) "yes" probably depended on who you were talking to. In the histories of the Sanhedrin presented in both the Jerusalem and Babylonian Talmuds, it's stated that in 30 AD the Sanhedrin, under the influence of its Hillelite Pharisee members (i.e., disciples of former Sanhedrin head Hillel, who was dead by that time), ruled that in view of all the limitations imposed by Jewish law on capital crimes proceedings--and the presumed intent behind those limitations--capital punishment was best seen as a "hypothetical" maximum penalty, ultimately unfit for flawed human judicial systems to mete out. This was precisely the argument used long before to abrogate the Deuteronomic injunction to kill a rebellious son, and in view of this, subsequent Jewish tradition has tended to interpret the Sanhedrin decision as effectively nullifying religious justification of the death penalty, despite not condemning it in abstract principle. Of course in all likelihood A) this was not a unanimous decision--probably some Shammaite Pharisees and Sadducees disagreed; B) its immediate impact was limited by the fact that they were ruling on a power they didn't then have anyway, making it politically inconsequential; and C) Judaism had no central theological authority, then or now, so in principle a Jew could--and can--support the death penalty in good conscience provided that s/he trusts the court rendering the verdict to be "perfectly" just.
I attended a Catholic high school for a couple years, and as I recall one of my teachers there suggested that the 'writing on the ground' was probably a way of underlining the idea that everyone has wrongs they might be held to account for (and in those days, likely a few which could've landed them in far more trouble than today if caught!). Who knows, but it seemed a plausible enough idea to me.
