Why was protectionism considered bad in the great depression? its said that "there is all-but-universal agreement that the adoption of restrictive trade policies was destructive and counterproductive and that similarly succumbing to protectionism in our current slump should be avoided at all cost."

However, doesn't that depend on your net exports? so if more is leaking from your economy, than is being injected, protectionism is a good strategy, right?
Net exports aren't in and of themselves a bad thing; they should just be a transitory phenomenon. A prolonged export imbalance should just lead to an exchange rate adjustment which fixes the imbalance.

Protectionism on the whole, in general, leaves all sides worse off. David Ricardo documented it very well; and it wouldn't surprise me if there were proverbs in all the major ancient civilisations to the same effect.

There are exceptions: for example, infant industries typically need protection during their vulnerable early years, so that they can reach sufficient maturity that they can thrive under international competition.

However, that's different to the general protectionism that does get proposed at times of deep slump - the Long Depression, the Great Depression, and the current (2008-now) slump: then, it's one of the (socially and economically) worst options, but does appeal to the masses when they're desperate for an "other" to blame, and for easy answers to complex questions.