The question the rude guy asked (that are the two men really the best on offer
) was a reasonable one
, but the context that he gave was completely out of order: conspiracy theorist and loaded with judgement 
I agree that he might (as he apparently claimed afterwards) still be undecided but leaning towards the Conservatives but he's actually undecided but leaning towards the Conservatives.
On the immigration point, they were both guilty of concentrating on the "boats" (many of whom might actually be legitimately claiming asylum) rather than legitimate immigration as a whole - which is 10 times larger than the numbers on the boats 
Last year, c1.6 million people came to the UK, and c500,000 left the UK, with a net migration figure of 685,000. Boat crossings amounted to c50,000 - I'm not sure they're even included in these figures 
And that's before you even get into why the legitimate immigrants came here (spoiler: we needed them
) and what immigrants can offer (spoiler: pre the EU referendum, analysis showed that EU immigrants were net contributors to the economy
). How else are we going to grow the economy? Growing the economy will itself help reduce the tax load.