aggregate.cs

// map

from p in docs.Payments
let fraud = p.isFraud != 0 || p.isFlaggedFraud != 0
select new {
    Count = 1,
    Customer = p.nameOrig,
    Transactions = new []{
      new {
          Type = p.type,
          To = p.nameDest,
          Amount = p.amount,
          MaxAmount = p.amount,
          Date = p.step,
          Fraud = fraud,
          Count = 1,
      }  
    },
    MaxAmount = p.amount,
    GoodDestinations = fraud ? new object[0] : new object []{p.nameDest},
    BadDestinations =  !fraud ? new object[0] : new object []{p.nameDest},
}

// reduce

from result in results 
group result by result.Customer into g
select new {
    Count = g.Sum(x=>x.Count),
    MaxAmount = g.Sum(x=>x.MaxAmount),
    Customer = g.Key,
    GoodDestinations = g.SelectMany(x=>x.GoodDestinations).Distinct(),
    BadDestinations = g.SelectMany(x=>x.BadDestinations).Distinct(),

    Transactions = g.SelectMany(x=>x.Transactions).GroupBy(x=>new { x.To, x.Type })
        .Select(gx => new {
            gx.Key.Type,
            gx.Key.To,
            Amount = gx.Sum(x=>x.Amount),
            Date = gx.OrderBy(x=>x.Date).Last(),
            Fraud = gx.Any(x=>x.Fraud),
            Count = gx.Sum(x=>x.Count),
            MaxAmount = gx.Sum(x=>x.MaxAmount),
        })
}