If we were able to somehow talk to poor Lord Randall himself, there's just one thing we'd say to warn him: in the immortal words of 90s rappers Bell Biv DeVoe, "That girl is poison!"Literally. Not only is poison a stand-in for the pains of love and betrayal, it's also a legit, non-metaphorical element here; through Lord Randall's fate, we begin to see that love itself can be poisonous, and even fatal. That is, if your main squeeze turns out to be a murderous wacko.
Line 15: When we first hear that Lord Randall's dogs have died under suspicious circumstances, we immediately prick up our ears. After all, up until then, there hasn't been any indication that Lord Randall has any enemies, so why this suggestion of foul play?
Line 17: Lord Randall's mother thinks just like us; she jumps from the dogs being poisoned to Lord Randall himself being poisoned. We're still operating on the literal level here.
Line 19: Finally, the symbolic meaning of Lord Randall's death by poison becomes apparent. He's sad because his "true love" poisoned him both in body and soul, which is to say, he's been slain by actual poison and by the metaphorical poison of betrayed love in equal measure.