The third statement is in Letter 153 in The Letters of JRR Tolkien, which is a draft letter. It's not clear if a letter based on this draft was ever sent, and if so whether or not it included this statement (or something similar).
I should regard them as no more wicked or foolish (but in much the same peril) as Catholics engaged in certain kinds of physical research (e.g. those producing, if only as by-products, poisonous gases and explosives): things not necessarily evil, but which, things being as they are, and the nature and motives of the economic masters who provide all the means for their work being as they are, are pretty certain to serve evil ends.
This makes it clear what Tolkien's own opinion is: poison is necessarily not in itself evil, but is almost certain to be used for evil purposes.